Move PG_MAX_AUTH_TOKEN_LENGTH to libpq/auth.h

Move PG_MAX_AUTH_TOKEN_LENGTH to libpq/auth.h

Future SASL mechanism, like OAUTHBEARER, will use this as a limit on
token messages coming from the client, so promote it to the header
file to make it available.

This patch is extracted from a larger body of work aimed at adding
support for OAUTHBEARER in libpq.

Author: Jacob Champion <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/CAOYmi+kJqzo6XsR9TEhvVfeVNQ-TyFM5LATypm9yoQVYk=4Wrw@mail.gmail.com