Add Base64.IsValid and allow Base64.DecodeXx methods to skip whitespace (#85938)

Add Base64.IsValid and allow Base64.DecodeXx methods to skip whitespace (#85938)

* Allow Base64Decoder to ignore space chars, add IsValid methods and tests

* Some cleanup of Base64.IsValid changes

This includes making FromBase64Transform significantly faster via SearchValues.

* Address PR feedback and some more cleanup

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Co-authored-by: Heath Baron-Morgan <heathbm@outlook.com>