Enhancement: Add support for TLS client certificate authentication.

Support has been added to the http backend for providing a TLS client
certificate/key pair using `--tls-client-cert` and `--tls-client-key` when
connecting to https resources that perform TLS client certificate
authentication.

https://github.com/restic/restic/issues/1522
https://github.com/restic/restic/pull/1524