support email login_hint

[mcalinghee]

It is now possible to forward the login_hint with a mxid to the upstream providers thanks to #4512

This PR allows us to support the login_hint with an email when login_with_email_allowed is activated.
In particular this PR will display the login_hint ie. email on the MAS login page.
It will also forward the email login_hint to upstream providers.

[sandhose]

I think it makes more sense to have the upper level handle this logic: always parse the email login hint, and let the caller decide what to do with it

[sandhose]

Please use a static RNG and the MockClock in tests instead

Suggested change

	#[allow(clippy::disallowed_methods)]
	let mut rng = thread_rng();
	#[allow(clippy::disallowed_methods)]
	let now = Utc::now();
	let now = MockClock::default().now();
	let mut rng = rand_chacha::ChaChaRng::seed_from_u64(42);

There is a reason for those methods are not allowed: we want tests to have a stable outcome, so that we can use snapshots when needed

[mcalinghee]

agreed but I would need to add a dependency on mas-storage on the data-model crate and this would end up as a cyclic dependency.
If this is ok, I would extract these static function into data-model to be able to use them in both mas-storage and data-model but there will be a bit of code refactor in this PR

[sandhose]

Hah, of course! If you feel like doing it, you can move Clock/MockClock/SystemClock/BoxClock/BoxRng to mas-data-model, either in a separate PR or in this one

I'm fine with keeping it like that if it's too annoying to refactor now

[mcalinghee]

Ok job done.

It was a bit annoying but I have done most of the refactoring in c314802 and brought minor corrections afterwards. I hope this is ok.

Hopefully, it will not be too painful to perform the review.
Let me know if you prefer to have this in another PR as the refacto is quite heavy.

[sandhose]

the logic would then be something like

let value = match grant.parse_login_hint(homeserver.homeserver()) {
    LoginHint::MXID(mxid) => Some(mxid.localpart().to_owned()),
    LoginHint::Email(email) if site_config.login_with_email_allowed => Some(email.to_string()),
    _ => None,
};

[sandhose]

@mcalinghee after an initial review, I would appreciate if you don't force-push to the branch, as it often makes GitHub confused and makes it harder to understand what changed. If you ever need to update from the main branch, prefer merging it into your branch instead of rebasing :)