Add a mu backend for gnus-search

Add a mu backend for gnus-search

* lisp/gnus-search.el (gnus-search-mu-program): New defcustom
(gnus-search-mu-switches): New defcustom
(gnus-search-mu-remove-prefix): New defcustom
(gnus-search-mu-config-directory): New defcustom
(gnus-search-mu-raw-queries-p): New defcustom
(gnus-search-mu): New subclass of gnus-search-indexed
(gnus-search-transform-expression): New method
(gnus-search-mu-handle-date): New function
(gnus-search-mu-handle-flag): New function
(gnus-search-indexed-extract): New method
(gnus-search-indexed-search-command): New method (bug#54662).