add sample for multi datasource (Azure#19655)

Author: zhichengliu12581

sdk/cosmos/azure-spring-data-cosmos/README.md

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+# Azure Spring Boot Sample Cosmos Multi Database Multi Account for Java
+
+## Key concepts
+## Getting started
+
+### Environment checklist
+We need to ensure that this [environment checklist][ready-to-run-checklist] is completed before the run.
+
+### Configure Cosmos Database
+1. Log into <https://portal.azure.com>.
+
+1. Click `Create a resource`.
+
+1. Input `Azure Cosmos DB`.
+
+1. Click `Azure Cosmos DB`
+    ![Find Cosmos Resource 01](resource/creating-cosmos-01.png)
+
+    ![Find Cosmos Resource 02](resource/creating-cosmos-02.png)
+
+1. Click **Create**.
+
+    ![Create new Cosmos](resource/creating-cosmos-03.png)
+
+1. On the **Create key vault** page, input `Subscription`, `Resource group`, `Account Name`, then click `Review + Create`.
+
+    ![Specify the options](resource/specify-the-options.png)
+
+    ![Create Cosmos resource](resource/create-cosmos-resource.png)
+
+1. When complete, click `Go to resource`.
+
+    ![Go to resource](resource/go-to-resource.png)
+
+1. Click **Keys** in the left navigation pane, copy your **URI**, the **PRIMARY KEY** and **SECONDARY KEY**;
+
+    ![Get Connect Info](resource/get-connect-info.png)
+
+1. Replace the content in `application.properties` with the obtained information.
+
+1. We need to create another cosmos DB as the secondary like the steps above.
+
+1. Add MYSQL connection attributes in `application.properties`.
+
+## Key concepts
+## Examples
+### Configure application.yml
+```yaml
+# primary account cosmos config
+azure.cosmos.primary.uri=your-primary-cosmosDb-uri
+azure.cosmos.primary.key=your-primary-cosmosDb-key
+azure.cosmos.primary.secondary-key=your-primary-cosmosDb-secondary-key
+azure.cosmos.primary.database=your-primary-cosmosDb-dbName
+azure.cosmos.primary.populate-query-metrics=if-populate-query-metrics
+
+# secondary account cosmos config
+azure.cosmos.secondary.uri=your-secondary-cosmosDb-uri
+azure.cosmos.secondary.key=your-secondary-cosmosDb-key
+azure.cosmos.secondary.secondary-key=your-secondary-cosmosDb-secondary-key
+azure.cosmos.secondary.database=your-secondary-cosmosDb-dbName
+azure.cosmos.secondary.populate-query-metrics=if-populate-query-metrics
+
+#mysql connection attributes
+spring.jpa.hibernate.ddl-auto=update
+spring.datasource.url=jdbc:mysql://${MYSQL_HOST:localhost}:3306/db_example
+spring.datasource.username=your-mysql-username
+spring.datasource.password=your-mysql-password
+```
+
+### Run with Maven
+```shell
+cd azure-spring-boot-samples/azure-spring-boot-sample-multi-database
+mvn spring-boot:run
+```
+
+Verify Result:
+The corresponding data is added to the mysql database and cosmos database
+    ![Result in MYSQL](resource/result-in-mysql.png)
+    ![Result in Primary Cosmos Database](resource/result-in-primary-cosmos-database.png)
+    ![Result in Secondary Cosmos Database](resource/result-in-secondary-cosmos-database.png)
+    
+## Troubleshooting
+## Next steps
+## Contributing
+
+<!-- LINKS -->
+[ready-to-run-checklist]: https://github.com/Azure/azure-sdk-for-java/blob/master/sdk/spring/azure-spring-boot-samples/README.md#ready-to-run-checklist

sdk/cosmos/azure-spring-data-cosmos/src/samples/java/com/azure/spring/data/cosmos/multidatasource/MultiDatasourceApplication.java

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-parent</artifactId>
+    <version>2.4.3</version> <!-- {x-version-update;org.springframework.boot:spring-boot-starter-parent;external_dependency} -->
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>com.azure.spring</groupId>
+  <artifactId>azure-spring-boot-sample-cosmos-multi-database-multi-account</artifactId>
+  <version>1.0.0</version> <!-- {x-version-update;com.azure.spring:azure-spring-boot-sample-cosmos-multi-database;current} -->
+  <name>azure-spring-boot-sample-cosmos-multi-database</name>
+  <description>Azure Spring Boot Sample Cosmos Multi Database</description>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-data-jpa</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>mysql</groupId>
+      <artifactId>mysql-connector-java</artifactId>
+      <scope>runtime</scope>
+    </dependency>
+    <dependency>
+      <groupId>com.azure</groupId>
+      <artifactId>azure-spring-data-cosmos</artifactId>
+      <version>3.5.0-beta.1</version> <!-- {x-version-update;com.azure:azure-spring-data-cosmos;current} -->
+    </dependency>
+  </dependencies>
+</project>