Enhancement: Add `--cacert` to specify TLS certificates to check against

We've added the `--cacert` option which can be used to pass one (or more) CA
certificates to restic. These are used in addition to the system CA
certificates to verify HTTPS certificates (e.g. for the REST backend).

https://github.com/restic/restic/issues/1114
https://github.com/restic/restic/pull/1276


