1728 - Enhance/Fix ByteArrayConverter to support base64 deserialization of byte[]

[brendonparker]

Issue #, if available: 1728

Description of changes:
Add support for deserializing a base64 string to a byte[].

If the token is a string, assume it is base64 encoded string and get bytes.

Otherwise, continue to do what it did before.

I added xunit tests for both conditions. To ensure my desired behavior works, and the existing behavior also works. I'm not strong in xunit, so if there are tweaks needed, please advise. Also wasn't sure where the best home/project for those tests might be.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[brendonparker]

One callout: serialization will still convert a byte[] to an array of numbers (ints) in json. No good way of supporting both write formats in one converter.

But in this approach, at least reading/deserializing can be flexible.

[normj]

Thanks for the fix. I agree there isn't really anything we can do for writes without some pre knowledge of what the format is supposed to be.

If you want bump the version in the project file to 2.4.3 that will save us from creating another version bump BR. https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.Serialization.SystemTextJson/Amazon.Lambda.Serialization.SystemTextJson.csproj#L12C10-L12C23