fix: Improve handling of primary keys with property converters with DisableFetchingTableMetadata

[ashovlin]

Description

When using DisableFetchingTableMetadata in the object-persistence programming model, the SDK infers its understanding of a DynamoDB table and/or index's key structure based on the attributed data model, as opposed to calling DescribeTable to load the actual table/index configuration.

The data type of primary keys is expected to be one of our DynamoDBEntryType and can be represented by a Primitive.

aws-sdk-net/sdk/src/Services/DynamoDBv2/Custom/DocumentModel/Primitive.cs

Lines 31 to 44 in 1b8348b

	public enum DynamoDBEntryType {
	/// <summary>
	/// DynamoDB String type.
	/// </summary>
	String,
	/// <summary>
	/// DynamoDB Numeric type.
	/// </summary>
	Numeric,
	/// <summary>
	/// DynamoDB Binary type.
	/// </summary>
	Binary
	}

The SDK was throwing an exception for the case when:

1. A property converter is applied to an attribute that is part of a key
2. That property converter returns an UnconvertedDynamoDBEntry (as opposed to a Primitive directly)

Now, if a property converter returns an UnconvertedDynamoDBEntry for a property used in a key, the SDK will attempt to convert it again towards a Primitive. If it still cannot, I clarified the exception that is thrown.

Motivation and Context

DOTNET-7524 -> #3323

Testing

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