support for mTLS auth with OpAMP server

[trentm]

Some interest in mTLS support was expressed in discussions.
IIRC the sample OpAMP server in opamp-go.git requires(?) or supports mTLS for auth.

Q: Are more Node.js TLS connection options that the single connect.ca option documented by @elastic/opamp-client-node required for good mTLS support?

(See slightly related for OTLP: #961
Only slightly related because that env var is spec'd by OTel.)

[trentm]

There are a couple other envvars for other mTLS config to support as well.