libpq-oauth: Use the PGoauthBearerRequestV2 API

libpq-oauth: Use the PGoauthBearerRequestV2 API

Switch the private libpq-oauth ABI to a public one, based on the new
PGoauthBearerRequestV2 API. A huge amount of glue code can be removed as
part of this, and several code paths can be deduplicated. Additionally,
the shared library no longer needs to change its name for every major
release; it's now just "libpq-oauth.so".

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/CAOYmi%2BmrGg%2Bn_X2MOLgeWcj3v_M00gR8uz_D7mM8z%3DdX1JYVbg%40mail.gmail.com