Support auth-source-pass in ERC

Support auth-source-pass in ERC

* doc/misc/erc.texi: Mention that the auth-source-pass backend is
supported.
* lisp/erc/erc-compat.el (erc-compat--29-auth-source-pass-search,
erc-compat--29-auth-source-pass--build-result-many,
erc-compat--29-auth-source-pass--retrieve-parsed,
erc-compat--29-auth-source-pass-backend-parse,
erc-compat--auth-source-backend-parser-functions): Adapt some yet
unreleased functions from auth-source-pass that mimic the netrc
backend, and add forward declarations to support them.

* lisp/erc/erc.el (erc--auth-source-search): Use own auth-source-pass
erc-compat backend.
* test/lisp/erc/erc-services-tests.el
(erc-join-tests--auth-source-pass-entries): Remove useless items.
(erc--auth-source-search--pass-standard,
erc--auth-source-search--pass-announced,
erc--auth-source-search--pass-overrides): Remove `ert-skip' guard.
(Bug#58985.)