Ability to add blobs for TLS

[joedborg]

https://bugs.launchpad.net/layer-docker-registry/+bug/1788891

[kwmonroe]

Lookin good, but I think we're missing a couple corner cases:

• _write_tls_blobs_to_files should return or block with a message about invalid config if tls-*-path config vars aren't truthy
• config should override the cert-provider relation, so in the reactive code, we should update the cert handlers so they only fire @when_not(config.set.tls-*-blob)
• need a new reactive handler @when(config.set.tls-*-blob) that sets charm.docker-registry.tls-enabled and calls configure. this is how clients know that registry comms should go over https.

[kwmonroe]

We can't return false in configure_registry since tls is optional. Also, we still need reactive handlers to ensure config overrides relation data.