Enhancement: Add new command `self-update`

We have added a new command called `self-update` which downloads the
latest released version of restic from GitHub and replaces the current
binary with it. It does not rely on any external program (so it'll work
everywhere), but still verifies the GPG signature using the embedded GPG
public key.

By default, the `self-update` command is hidden behind the `selfupdate`
built tag, which is only set when restic is built using `build.go` (including
official releases). The reason for this is that downstream distributions will
then not include the command by default, so users are encouraged to use the
platform-specific distribution mechanism.

https://github.com/restic/restic/pull/1949
