gapic: consolidate routing header parsing/injection

[noahdietz]

The google.api.routing annotation can be configured to parse out portions of a field's value for injection into the x-goog-request-params. Originally implemented in #887, a few improvements are possible:

• Consolidate application of regexes and header building into gax-go. A similar endeavor was achieved in gax-php.
• initialize regexes on init rather than on every method invocation

[codyoss]

We should try to remove the use of regexp compile from the hotpath of api calls as this could be a concern for performance sensitive apis like a GCS read. Have not thought through or looked at the viability of this, but something to investigate. Seems like we could at minimum make this a sunken at package load cost by making these private globals.