Add PAM Tests and Option For Custom PAM Config Location

Hello,

I noticed that there are currently no tests for the PAM authentication
functionality in postgres so I came up with the attached patch to add
some. I suspect that no tests were added previously because it would
have involved using root privileges to set up the environment for the
test or just rely on the individual set up the root environment before
running the test similar to
`src/interfaces/libpq/t/004_load_balance_dns.pl` [0] and then adding
validations to skip if the environment wasn't set up.

This patch allows the PAM tests to be set up and run without root by
use of the `pam_start_confdir`[1] function that was added in linux-pam
1.4.0 [2]. This function is essentially the same as `pam_start` but
has one extra parameter that allows you to either pass NULL for the
same behavior as `pam_start` or pass a char * to the location of an
alternative location for `/etc/pam.d`. This functionality is exposed
via an added `pamconfdir` option that can be used in the pg_hba.conf
file after `pam` method is specified. This option is conditionally
available via macros on the absence or presence of the
`pam_start_confdir` function. I had to make the use of this function
conditional because its use in linux-pam is recent enough to where
some older LTS linux distros will not support it. For example rocky8
uses PAM 1.3.1 which does not container `pam_start_confdir`, rocky9
uses PAM 1.5.1 which will support it.

The tests are skipped if:
- postgres was not built with PAM
- postgres was not build with a PAM that has the `pam_start_confdir` function
- PG_EXTRA_TESTS does not include pam (similar to how LDAP and GSSAPI
tests are set up)

The tests set up a temporary pam configuration folder and only use
pamconfdir in hba. Though this means that we cannot test a non
existent pamconfdir in the HBA file I think this is still better than
no testing at all as the code paths are mostly the same.

Multiple pamservices are tested using `pam_permit` [3] and `pam_exec`
[4] pam modules. These modules are both built into the linux-pam
source tree. `pam_permit` just allows everything, more fine grained
testing can be done with `pam_exec` which essentially passes the
password, username, etc to a custom made bash script and acceptance or
rejection of the auth request essentially comes down to a zero or non
zero return from that script.

I have tested this functionality using the make build system and using
the meson build system. I tested on nixos with both pam 1.3.1 and
1.7.2 so that I know it builds both with and without this function.

Happy to make any changes and appreciate any feedback.

Thanks,
Andrew Jackson

[0] https://github.com/postgres/postgres/blob/ec13f71049cda06ea55a67783b822d98824f4d15/src/interfaces/libpq/t/004_load_balance_dns.pl
[1] https://man7.org/linux/man-pages/man3/pam_start.3.html
[2] https://github.com/linux-pam/linux-pam/blob/master/NEWS
[3] https://man7.org/linux/man-pages/man8/pam_permit.8.html
[4] https://linux.die.net/man/8/pam_exec