CDRIVER-6134 check SASL username length

[kevinAlbs]

Summary

Check SASL username length. See CDRIVER-6134 and linked tickets for more information.

Background

Cyrus-SASL 2.1.28 documents expected behavior of the callback:

int sasl_canon_user_t(sasl_conn_t *conn, void *context, const char *user, unsigned ulen,
                      unsigned flags, const char *user_realm, char *out_user,
                      unsigned out_umax, unsigned *out_ulen)

out_umax – Maximum length for out_user

Cyrus-SASL source suggests out_user is a buffer of size out_umax+1. However, since this does not appear clearly documented, this implementation assumes a size of out_umax.

[vector-of-bool]

LGTM