pgsql: 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 <jacob(dot)champion(at)enterprisedb(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/CAOYmi+kJqzo6XsR9TEhvVfeVNQ-TyFM5LATypm9yoQVYk=4Wrw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e21d6f297158db1383a7c9a668ebe1048f2eac39

Modified Files
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src/backend/libpq/auth.c | 16 ----------------
src/include/libpq/auth.h | 16 ++++++++++++++++
2 files changed, 16 insertions(+), 16 deletions(-)