Namespace outer attribute support for #[derive(WpDerivedRequest)]

[oguzkocer]

This PR adds support for defining an endpoint namespace for #[derive(WpDerivedRequest)]:

#[derive(WpDerivedRequest)]
#[Namespace("/wp/v2")]
#[SparseField(SparseUserField)]
enum UsersRequest {
    #[contextual_get(url = "/users", params = &UserListParams, output = Vec<SparseUser>)]
    List,
}

Up until this PR, we were using /wp/v2 for all endpoints, hardcoded into the WP_JSON_PATH_SEGMENTS constant. However, the WordPress REST endpoints have different routes, for example Wp Site Health Tests are accessed through /wp-site-health/v1/.

The new Namespace outer attribute takes a single literal token, splits it by / character and extends the base url by appending its parts. This should allow us to describe any route we need.

To Test

• make test-server && make dump-mysql && make backup-wp-content-plugins
• cargo test

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Note that this PR is assigned to 0.1 milestone, but not because it needs to be included in 0.1, but rather because 0.1 is not cut yet. If the release ends up being cut before merging this PR, we can update it to 0.2.