googlec2p: use the bootstrap parsing code to generate parsed bootstrap config instead of handcrafting it

[easwars]

The google_c2p resolver handcrafts bootstrap configuration directly into the parsed bootstrap.Config struct. This should be changed to write out the JSON contents, thereby letting the bootstrap parsing code handle the actual parsing and the building of the bootstrap.Config struct.

This will ensure that changes made to the bootstrap parsing code to support xDS client fallback will be available to the google_c2p resolver.