Add non-IRCv3 SASL module to ERC

Add non-IRCv3 SASL module to ERC

* doc/misc/erc.texi: Add SASL section in Advanced Usage chapter to
document the new SASL module.

* etc/ERC-NEWS: Mention addition of erc-sasl module for SASL support.

* lisp/erc/erc-compat.el
(erc-compat--29-sasl-scram-construct-gs2-header,
erc-compat--29-sasl-scram-client-first-message,
erc-compat--29-sasl-scram--client-final-message): Fix encoding bug and
add minimal authorization support with copies of SASL functions
introduced in Emacs 29.

* lisp/erc/erc.el (erc-modules): Add `sasl'.
* lisp/erc/erc-sasl.el: New file (bug#29108).
* test/lisp/erc/erc-sasl-tests.el: New file.
* test/lisp/erc/erc-scenarios-sasl.el: New file.
* test/lisp/erc/resources/sasl/plain-failed.eld: New file.
* test/lisp/erc/resources/sasl/plain.eld: New file.
* test/lisp/erc/resources/sasl/scram-sha-1.eld: New file.
* test/lisp/erc/resources/sasl/scram-sha-256.eld: New file.
* test/lisp/erc/resources/sasl/external.eld: New file.