pki: Implement API endpoints for certs and caddy trust (#4443)

* admin: Implement /pki/certificates/<id> API

* pki: Lower "skip_install_trust" log level to INFO

See https://github.com/caddyserver/caddy/issues/4058#issuecomment-976132935

It's not necessary to warn about this, because this was an option explicitly configured by the user. Still useful to log, but we don't need to be so loud about it.

* cmd: Export functions needed for PKI app, return API response to caller

* pki: Rewrite `caddy trust` command to use new admin endpoint instead

* pki: Rewrite `caddy untrust` command to support using admin endpoint

* Refactor cmd and pki packages for determining admin API endpoint

caddy.go

@@ -427,6 +427,13 @@ func run(newCfg *Config, start bool) error {
 		return nil
 	}
 
+	// Provision any admin routers which may need to access
+	// some of the other apps at runtime
+	err = newCfg.Admin.provisionAdminRouters(ctx)
+	if err != nil {
+		return err
+	}
+
 	// Start
 	err = func() error {
 		var started []string