Decoder: Remove use of global name validation and add validation (#808)

This change removes the use of the global variable to determine name validation, using the provided format instead.

Also:

* adds validity checks to the text parser, which previously did not check for name validity.
* Returns error if the caller tries to create a decoder for prototext format, which is not supported.

Signed-off-by: Owen Williams <owen.williams@grafana.com>