[App Config] FeatureFlag (and SecretReference) redesign - version 8.1 (Azure#15193)

Based on option 8 at https://gist.github.com/HarshaNalluru/d4d9aa16c89eaceb50a005df800bf4ea

Sample https://github.com/Azure/azure-sdk-for-js/blob/6723c01b83a888b02c4f6743c33d0d43dc26558e/sdk/appconfiguration/app-configuration/samples-dev/featureFlag.ts

### TODO
- [x] New design for 
  - [x] feature flag
  - [x] secret reference 
- [x] Api Report
- [x] update tests for both, and fix playback
- [x] samples for both
   - [x] samples-dev
   - [x] regenerate docs.microsoft.com samples
- [x] update readmes accordingly
- [x] update react sample
- [x] changelog

Author: HarshaNalluru

common/config/rush/common-versions.json

@@ -70,6 +70,8 @@
     "@azure/ms-rest-nodeauth": ["^0.9.2"],
     // Idenity is moving from v1 to v2. Moving all packages to v2 is going to take a bit of time, in the mean time we could use v2 on the perf-identity tests.
     "@azure/identity": ["2.0.0-beta.3", "^1.1.0"],
+    // App Config uses keyvault-secrets in a sample, switch to latest once the preview becomes GA
+    "@azure/keyvault-secrets": ["^4.1.0", "^4.2.0-beta.4"],
     // Issue #14771 tracks updating to these versions
     "@microsoft/api-extractor": ["7.13.2"],
     "prettier": ["2.2.1"]

common/config/rush/pnpm-lock.yaml

@@ -3,6 +3,10 @@
 ## 1.2.0-beta.2 (Unreleased)
 
 - With [#15136](https://github.com/Azure/azure-sdk-for-js/pull/15136), if the key of a feature flag(setting with `contentType="application/vnd.microsoft.appconfig.ff+json;charset=utf-8"`) doesn't start with `".appconfig.featureflag/"` (featureFlagPrefix), SDK adds the prefix before sending the request.
+- New design for feature flags and secret references,
+  - New types for FeatureFlag and SecretReference - `ConfigurationSetting<FeatureFlagValue>` and `ConfigurationSetting<SecretReferenceValue>`
+  - Upon using `getConfigurationSetting`(or add/update), use `parseFeatureFlag` and `parseSecretReference` methods to access the properties(to translate `ConfigurationSetting` into the types above).
+  - Helper method `isFeatureFlag` (and `isSecretReference`) checks the contentType and return boolean values.
 
 ## 1.2.0-beta.1 (2021-04-06)
 

sdk/appconfiguration/app-configuration/CHANGELOG.md

@@ -96,7 +96,7 @@
     "@azure/dev-tool": "^1.0.0",
     "@azure/eslint-plugin-azure-sdk": "^3.0.0",
     "@azure/identity": "2.0.0-beta.3",
-    "@azure/keyvault-secrets": "^4.2.0-beta.4",
+    "@azure/keyvault-secrets": "^4.1.0",
     "@azure/test-utils-recorder": "^1.0.0",
     "@microsoft/api-extractor": "7.7.11",
     "@rollup/plugin-commonjs": "11.0.2",