ConfigServer+LibConfig: Add pledge_domains() API

This API lets applications specify which configuration domains they
will be accessing throughout their lifetime. It works similarly in
spirit to the kernel's pledge().

You cannot pledge_domains() more than once, and once you have used it,
it's no longer possible to access any other configuration domain.

This is obviously just a first cut of this mechanism, and we may need
to tweak it further as we go.

Userland/Libraries/LibConfig/Client.cpp

@@ -19,6 +19,11 @@ Client& Client::the()
     return *s_the;
 }
 
+void Client::pledge_domains(Vector<String> const& domains)
+{
+    async_pledge_domains(domains);
+}
+
 String Client::read_string(StringView domain, StringView group, StringView key, StringView fallback)
 {
     return read_string_value(domain, group, key).value_or(fallback);