feat(client): add useSystemCa and combine it with trustedCaFile (#5572)

Move the frpc-only TLS option into the shared TLSConfig as
transport.tls.useSystemCa, replacing the unpublished useSystemRoots.
When set, frpc verifies the server with the system CAs; when
trustedCaFile is also set, its CAs are appended to the system pool so
both are trusted. Failures to load either source abort the connection
instead of falling back. Enabling the option without effective TLS is a
configuration error, and the field is rejected in frps transport.tls
and in both webServer.tls sections.

Fixes #5545

pkg/config/v1/validation/common.go

@@ -23,6 +23,9 @@ import (
 
 func validateWebServerConfig(c *v1.WebServerConfig) error {
 	if c.TLS != nil {
+		if c.TLS.UseSystemCa {
+			return fmt.Errorf("webServer.tls.useSystemCa is only supported in frpc transport.tls")
+		}
 		if c.TLS.CertFile == "" {
 			return fmt.Errorf("tls.certFile must be specified when tls is enabled")
 		}