Support IAsyncEnumerable

[sudoudaisuke]

Description

Add netstandard2.1 to TargetFrameworkds.
Add PachageReference Microsoft.Bcl.AsyncInterfaces to netstandard 2.0.
Implement IAsyncEnumerable for AsyncSearch on netstandard 2.0 and netstandard 2.1.

Motivation and Context

• For 'await foreach'. Support IAsyncEnumerable at DynamoDBv2.DataModel.AsyncSearch #1468
• support interface for mock. DynamoDb DocumentModel difficult to create stubs #604, Add interfaces for classes with internal constructors as returned by IDynamoDBContext methods #632, Added interfaces for AsyncSearch and AsyncSearch.Async and updated us… #1187, Add AsyncSearch interface for easier mocking #1391

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[matteo-prosperi]

Hello,
thank you for your PR.
Unfortunately we would not be able to add dependency on a new package or add a new target through a PR due to backward compatibility concerns. This will require more complex coordination and structural work across the SDK.
Nonetheless, this is great input and we will consider this in the context of some larger work we are doing in the SDK.
I am closing this for now.

[say25]

@matteo-prosperi any hints on when native support for this feature could be implemented?