Add option to specify Certificate Authority for metal-apiserver endpoint

[l0wl3vel]

Description

Add a flag+env variable to pin a Certificate Authority to validate apiserver identity against.

Definition of Done

• Wire up CLI+Env Var+field in metal-cli config file
• Document flag and usage

[majst01]

During client creation a RoundTripper can be passed:
https://github.com/metal-stack/api/blob/main/go/client/conn.go#L43