Support using auth-source for NickServ passwords in ERC

Support using auth-source for NickServ passwords in ERC

* lisp/etc/erc-services.el (erc-nickserv-passwords): Document that
the passwords are only used when erc-prompt-for-nickserv-password
is nil.
* (erc-use-auth-source-for-nickserv-password): New customizable
variable to enable checking auth-source for NickServ passwords.
* (etc-nickserv-get-password): New function to handle the lookup
of the NickServ password from both auth-source and the
erc-nickserv-passwords variable.
* (erc-nickserv-call-identify-function): Use new
erc-nickserv-get-password function to lookup NickServ passwords.
* (erc-nickserv-identify-autodetect, erc-nickserv-identify-on-connect,
erc-nickserv-identify-on-nick-change): Call
erc-nickserv-call-identify-function when
erc-use-auth-source-for-nickserv-password is set.
* etc/NEWS: Document change (bug#45340).