Extend Utf8JsonReader.CopyString to also accept JsonTokenType.Number values #84375
Description
The current implementation of the Utf8JsonReader.CopyString methods only accepts JSON string tokens, that is JsonTokenType.PropertyName and JsonTokenType.String. When implementing JsonConverters for large numeric types such as BigInteger (#60780) or Int128/UInt128 (#73500) extending the method to also handle JSON number tokens would greatly simplify getting allocation-free views of underlying JSON number encodings.