Add Base64 APIs to Utf8JsonReader, Utf8JsonWriter, and JsonElement (dotnet/corefx#38048)
* Add Utf8JsonWriter Base64 APIs.
* Add Utf8JsonReader Base64 APIs.
* Add JsonElement Base64 APIs.
* Update API shape based on review.
* Auto-generate the ref assembly.
* Address PR feedback so far.
* Add escaping step and update length counters accordingly.
* Add JsonWriter API tests.
* Add JsonReader and JsonElement tests.
Commit migrated from https://github.com/dotnet/corefx/commit/
82408cd90f4d4573d502e8df2ca437b35e6a37f7