attestation: fixing the segment attestationProvider to be attestationProvider which matches the API (Azure#16355)

Using the API Version in the Portal (2020-10-01):

Using API Version 2020-10-01 and a valid AttestationProvider with this endpoint returns an error that the `attestationProvider` resource type cannot be found:

```
$ az rest --method get --uri "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-dev/providers/Microsoft.Attestation/attestationProvider/tomdevaccprov/privateEndpointConnections?api-version=2020-10-01"
Not Found({"error":{"code":"InvalidResourceType","message":"The resource type could not be found in the namespace 'Microsoft.Attestation' for api version '2020-10-01'."}})
```

Updating this to `attestationProviders` fixes this (and correctly returns a 404 that there is no PrivateEndpointConnection for this Attestation Provider):

```
$ az rest --method get --uri "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-dev/providers/Microsoft.Attestation/attestationProviders/tomdevaccprov/privateEndpointConnections?api-version=2020-10-01"
Not Found({"error":{"code":"InvalidOperation","message":"PrivateEndpointConnection is not found."}})
```

---

Using API Version 2021-06-01-preview and a valid AttestationProvider with this endpoint returns an error that the `attestationProvider` resource type cannot be found:

```
$ az rest --method get --uri "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-dev/providers/Microsoft.Attestation/attestationProvider/tomdevaccprov/privateEndpointConnections?api-version=2021-06-01-preview"
Not Found({"error":{"code":"InvalidResourceType","message":"The resource type could not be found in the namespace 'Microsoft.Attestation' for api version '2021-06-01-preview'."}})
```

Updating this to `attestationProviders` fixes this (and correctly returns a 404 that there is no PrivateEndpointConnection for this Attestation Provider):

```
$ az rest --method get --uri "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-dev/providers/Microsoft.Attestation/attestationProviders/tomdevaccprov/privateEndpointConnections?api-version=2021-06-01-preview"
Not Found({"error":{"code":"InvalidOperation","message":"PrivateEndpointConnection is not found."}})
```

Author: tombuildsstuff

specification/attestation/resource-manager/Microsoft.Attestation/preview/2021-06-01-preview/attestation.json

@@ -267,7 +267,7 @@
         }
       }
     },
-    "/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Attestation/attestationProvider/{providerName}/privateEndpointConnections": {
+    "/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Attestation/attestationProviders/{providerName}/privateEndpointConnections": {
       "get": {
         "tags": [
           "PrivateEndpointConnections"

specification/attestation/resource-manager/Microsoft.Attestation/stable/2020-10-01/attestation.json

@@ -267,7 +267,7 @@
         }
       }
     },
-    "/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Attestation/attestationProvider/{providerName}/privateEndpointConnections": {
+    "/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Attestation/attestationProviders/{providerName}/privateEndpointConnections": {
       "get": {
         "tags": [
           "PrivateEndpointConnections"