pgsql: libpq: Allow developers to reimplement libpq-oauth

libpq: Allow developers to reimplement libpq-oauth

For PG19, since we won't have the ability to officially switch out flow
plugins, relax the flow-loading code to not require the internal init
function. Modules that don't have one will be treated as custom user
flows in error messages.

This will let bleeding-edge developers more easily test out the API and
provide feedback for PG20, by telling the runtime linker to find a
different libpq-oauth. It remains undocumented for end users.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/09532b4040ed4c313351366166f55e810f152d6a

Modified Files
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src/interfaces/libpq/fe-auth-oauth.c | 71 +++++++++++++++++++++---------------
src/interfaces/libpq/fe-auth-oauth.h | 2 +-
2 files changed, 42 insertions(+), 31 deletions(-)