tag: add tag.gpgSign config option to force all tags be GPG-signed

tag: add tag.gpgSign config option to force all tags be GPG-signed

As many CI/CD tools don't allow to control command line options when
executing `git tag` command, a default value in the configuration file
will allow to enforce tag signing if required.

The new config-file option tag.gpgSign is added to define default behavior
of tag signings. To override default behavior the command line option -s,
--sign and --no-sign can be used:

    $ git tag -m "commit message"

will generate a GPG signed tag if tag.gpgSign option is true, while

    $ git tag --no-sign -m "commit message"

will skip the signing step.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>