[feature] support CRL (certificate revocation list) files

### Preface
Nomad is currently using two systems for securing its protocols.
* ACL (which has a token revocation mechanism) controls access to data and APIs
* mTLS that could work in an untrusted #environment and even prevent other services from masquerading as Nomad agents.

### Use case
We have to use nomad agents over the internet and would like to protect our communication channels. So, mTLS sounds like a very natural choice. However, some of the agents could be compromised and we would like to be able to revoke their certificates.
Supporting a revocation list could help to avoid building a system for rotation low TTL certs.

What do you think?

P. S. And Vault already has a CRL support.

The issue names no files, tests, or entry points. Start by reviewing Nomad's existing mTLS configuration and how Vault's CRL support could relate to it. Done should mean agents can use certificate revocation lists to revoke compromised certificates without relying on short-lived certificate rotation.

Written by the indexing model from the issue text.