FR: add jj log --show-signature

[danyspin97]

As far as I know there is no way to show the signed commits when doing jj log. git provides --show-signature arguments to the log command, which is very useful for checking if all latest commits have been signed before pushing.

[yuja]

There's a WIP PR, #3141, which I think is revisited recently. iirc, we'll add some stock templates that include cryptographic signature information.