server: allow user to provide their own TLS cert and key

[dnephin]

Currently we use acme/autocert to accept either a cert from https://letsencrypt.org/, or generate one on the first request if a let's encrypt cert is not found.

Many orgs already have a chain of trust setup, and will likely want to provide a cert they've already generated to the infra server. Technically they could add the cert and key to the directory specified by tlsCache, but that's an undocumented interface, and probably not the one we want to support as a public interface.

We should have a way for the user to specify their own CA, cert, and private key. Adding something like this to the server options should work to allow the user to specify their own certs:

type TLSConfig struct {
    CA string
    Certificate string
    PrivateKey string
}

Where each field is the path to a file. We might not need CA initially, and might be fine with just cert and key. The CA would allow us to do mTLS if the client or connector also passed a certificate signed by the same CA.

From these options we can construct a tls.Config, and use that tls.Config in the http.Server.

[dnephin]

Done in #2398, follow up work will be part of #2362