Fix LDAP tls_options not working

What does this MR do and why?

This backports !122789 (merged) to the 16-0-stable-ee branch.

The removal of the Settingslogic gem in !113040 (merged) created a regression when LDAP tls_options were used. It resulted in Gitlab::config.ldap.servers.main.tls_options.cert returning nil instead of the certificate.

This occurred because Gitlab::Auth::Ldap::Config#custom_tls_options attempts to duplicate the tls_options Hash and symbolize the keys, but instead it altered the internal Hash representation of GitlabSettings::Options. Since all the keys were transformed to symbols but GitlabSettings::Options converts all keys to strings, the values were inaccessible.

To fix this, implement a dup to create a copy of the internal representation. We should also consider refactoring Gitlab::Auth::Ldap::Config.

Relates to #413017 (closed)

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