Replace &Option<T> with &T for UserListParams & PluginListParams

[oguzkocer]

Replaces &Option<UserListParams> with &UserListParams & &Option<PluginListParams> with &PluginListParams.

All fields in UserListParams & PluginListParams are optional, so this is mostly a syntax sugar change. I think using Option<&T> for these types have better semantics, because it tells the user that this params might or might not be there and if it's there, it's a reference to it. However, we are unable to use that with uniffi and have to opt for &Option<T> which has both different semantics and different handling rules for it.

I had reluctantly implemented it with &Option<T> back when I was first working on this, but it has been bugging me all this time. Now that I am working on generating these endpoints and request builders, this became more than a minor annoyance as I'd have to juggle between &Option<T> and Option<&T> from request builder to endpoint types.

Using UserListParams & PluginListParams is a good middle ground solution for this and will let me work on generating the request builders without the extra friction.

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Somewhat unrelated to this PR, I've added test cases for empty fields for filter users & filter plugins. I was just trying to make sure we were handling empty UsersListParams & PluginListParams and ended up adding these cases. Surprisingly, I realized that the server will return all fields if the _fields= is empty. I am not entirely sure if we are handling this case properly at the moment, but I think adding these tests will help make sure we pay attention to it and maybe iterate on it at a later time.

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To Test

• make test-server && make dump-mysql && make backup-wp-content-plugins
• cargo test --test '*' -- --nocapture --test-threads 1
• cd native/kotlin && ./gradlew :api:kotlin:integrationTest cAT