ads-client: Use AdsClientUrl consistently

[bendk]

I'm working on a new UniFFI parser
(mozilla/uniffi-rs#2841) and it currently requires that the type name in exported functions match the one used in the custom_type! macro.

I could probably rework the parser to handle the type alias, but this way feels cleaner to me anyways. Feel free to push back if you don't like this change though.

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