enhance: support client certificates for mutual TLS (#1273)

Add `runner.client_cert_file` and `runner.client_key_file` for Gitea instances that require mutual TLS. The runner presents the certificate on its RPCs, job summary uploads, and the action, reusable workflow and `builtin:checkout` clones from the instance. The files are read on use, so renewed certificates apply without a restart. Job steps do not get them. An incomplete or mismatched pair fails startup.

Clones now decide the token, client certificate and `runner.insecure` with one host check, which also gives reusable workflow clones the TLS settings they lacked. `runner.insecure` still only covers the registered address.

Fixes: https://gitea.com/gitea/runner/issues/321
Co-authored-by: silverwind <[email protected]>
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1273
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

act/runner/step_action_builtin.go

@@ -49,6 +49,8 @@ func (sab *stepActionBuiltin) main() common.Executor {
 			NoSkipCheckout:  rc.Config.NoSkipCheckout,
 			UseGitIgnore:    rc.Config.UseGitIgnore,
 			InsecureSkipTLS: rc.Config.InsecureSkipTLS,
+			ClientCertFile:  rc.Config.ClientCertFile,
+			ClientKeyFile:   rc.Config.ClientKeyFile,
 			Env:             sab.env,
 		})
 	})