Stop hard-coding protobuf enums in speech Encoding

[dhermes]

See #3288

/cc @pbogle

[lukesneeringer]

Really we should make this change everywhere that we surface a GAPIC enum.

[pbogle]

Can you help me understand why we don't simply expose the underlying GAPIC enum??

(If the challenge is to come up with the equivalent string representation for error messages, there are definitely ways to do that programatically.)

[dhermes]

@pbogle Looking at the class there is absolutely no reason.

[lukesneeringer]

We now expose the GAPIC enum. Problem solved. 🎉