Pass through search options for GitHub::Ldap::Domain#user?

Prerequisite for https://github.com/github/github/pull/34381. This allows clients to specify additional search options when querying for a user. The test demonstrates how a client can limit the attributes returned.

cc @github/ldap @mtodd 
Is the separate `options` vs `search_options` hash needed? Seems like it'd be fine to combine them?