fix: Delay the cast from IAmazonDynamoDB to AmazonDynamoDBClient in the DocumentModel

[ashovlin]

Description

Before: When initializing the document model Table object with an IAmazonDynamoDB, .NET/Standard/Core, we cast it right away to an AmazonDynamoDBClient.

• Because we didn't have an synchronous HTTP client, we removed the public sync apis from the low-level service client.
• However we never removed the synchronous public Table APIs, which still rely on the internal sync low-level APIs internally.
• So to work around this, we kept the sync low-level APIs as internal on AmazonDynamoDBClient. This is why we need the cast, since the sync low-level APIs are no longer part of the public IAmazonDynamoDB.

Now: We delay the cast until right before we need to access the internal, low-level sync APIs.

• For users still calling the sync APIs in .NET/Standard/Core, you can avoid this by moving to the async equivalents where possible.
• If you're still relying on the sync APIs but aren't providing an actual AmazonDynamoDBClient, you'll now see a clearer InvalidOperationException instead of the NullReferenceException from the failed cast.

Future:

1. We can deprecate the Table sync-over-async APIs entirely. This would remove the need for this cast.
2. Since .NET 5 added back a sync HTTP client, we've discussed internally re-exposing sync low-level APIs where possible.

Motivation and Context

DOTNET-6755 and #1589 (most of the way, until we do on of the Future ideas from above)

This allows Table consumers to mock the IAmazonDynamoDB from .NET/Standard/Core as long as you stick to async public APIs.

Testing

 dotnet test .\sdk\src\Services\DynamoDBv2\DynamoDBv2.sln
dotnet test .\sdk\AWSSDK.CoreAndCustomUnitTests.NetStandard.sln

PENDING - .NET Standard integ tests for DDB

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

[normj]

A couple minor nits but change looks good.

[normj]

nit: You can move the casting logic outside of the loop and only do it once.

[ashovlin]

Fixed in aa1e327

[normj]

nit: Move the casting logic above the loop.

[ashovlin]

Fixed in aa1e327

[ashovlin]

Realized that I was using too wide of a condition AWS_ASYNC_API, which was casting unnecessarily for .NET Framework 4.7.2 even though it still has sync APIs we can use. Switched to NETSTANDARD in aa1e327

This also made some .NET Framework tests invalid, so I removed them.

[ashovlin]

Dry-run e8acaff2-7604-4ade-a913-8ea4cc934cfe passed after adding 2f8a80e