interior mutability pattern for UserStore concrete implementations

[comprido96]

Proposal: refactor UserStore add_user method signature to take an immutable reference to self, i.e. &self instead of &mut self,
moving the RwLock from UserStore trait into concrete implementations:

app_state.rs:
#[derive(Clone)]
pub struct UserStoreType(pub Arc<Box>);

hashmap_user_store.rs:
#[derive(Default)]
pub struct HashmapUserStore {
users: RwLock<HashMap<String, User>>,
}

See last commit: comprido96@1f7e79c#diff-afd280d500d88b598001a76068239f3fa07150812b4f0883d3ce73e697f8ef57