cmake: Teach "-E tar" command a "--format=" option

Allows specifying a libarchive defined archive format currently restricted to
7zip, gnutar, pax, paxr and zip.

The default is "paxr" (pax restricted).

Help/manual/cmake.1.rst

@@ -215,6 +215,10 @@ Available commands are:
     names start in ``-``.
   ``--mtime=<date>``
     Specify modification time recorded in tarball entries.
+  ``--format=<format>``
+    Specify the format of the archive to be created.
+    Supported formats are: ``7zip``, ``gnutar``, ``pax``,
+    ``paxr`` (restricted pax, default), and ``zip``.
 
 ``time <command> [<args>...]``
   Run command and return elapsed time.

Help/release/dev/tar-write-format.rst

@@ -0,0 +1,6 @@
+tar-write-format
+----------------
+
+* The :manual:`cmake(1)` ``-E tar`` command learned a new
+  ``--format<format>`` option to specify the archive format to
+  be written.