Custom oauth validator options

Hello hackers,

The current OAuth code allows validators to add custom validation
logic, but does not allow them to introduce custom
authentication-related parameters in the place where they belong:
pg_hba.conf. As a workaround, both pg_oidc_validator and
postgres-keycloak-oauth-validator (and likely others; these are simply
the two I know of) rely on GUC variables instead.

I see two issues with this:

1. Configuration for OAuth validation ends up split across two
locations: issuer/scope and a few other parameters are defined in
pg_hba.conf, while custom parameters must be set in postgresql.conf.

2. We have received multiple questions asking how to configure
multiple OIDC servers with different parameter sets. I am not sure how
common it is to use multiple OAuth providers with a single PostgreSQL
instance, but the question is certainly reasonable.

Given this, I would like to ask what you think about making
pg_hba.conf extensible. At present, option parsing is hardcoded in
parse_hba_auth_opt, and any unknown parameter triggers an error at the
end of the function.

I can see a few possible approaches:

a. Add an OAuth-specific hook that allows injecting additional
option-parsing logic into this function, as part of the existing
OAuthValidatorCallbacks. This could be scoped to the validator used on
the specific HBA line, even if multiple validators are loaded.
b. Allow the existing startup callback to supply a list of additional
valid configuration names, with the validation callback responsible
for parsing and validating them.
c. Add a more generic hook usable by any extension. I do not currently
have concrete use cases outside OAuth, but perhaps others do.

I would be interested in your thoughts on whether an improvement in
this area would be useful.

I also have two related questions, which might be addressed as part of
the above or independently:

1. HBA options are parsed sequentially. If validator-specific options
are tied to a particular validator, this implies that validator=...
must appear before its parameters when multiple validators are loaded,
since we cannot otherwise determine which validator is used. Is this
acceptable behavior, or should options be allowed in any order?

2. If a validator introduces many options, an HBA line could become
very long and hard to read. Would it make sense to allow loading the
parameters for a given line from a separate file? This might already
be useful today: for example, if a long issuer URL is repeated across
several HBA lines, it could instead be defined once in an external
file and referenced multiple times.