Make `x-saasify-proxy-secret` optional

[amaury1093]

Right now, all requests going through this backend must have a header x-saasify-proxy-secret set to the value of the RCH_SAASIFY_SECRET env variable (=="reacher_dev_secret" by default). This is used by https://reacher.email, to only allow authenticated backend-to-backend requests.

I propose to make this x-saasify-proxy-secret check optional:

• if RCH_SAASIFY_SECRET env variable, then check x-saasify-proxy-secret header
• if not, then don't check x-saasify-proxy-secret

The idea is to make the following warp Filter optional, depending if the RCH_SAASIFY_SECRET is set or not:

backend/src/routes/check_email/header.rs

Lines 39 to 51 in 41e46bf

	/// Warp filter to check that the header secret is correct. We accept headers
	/// for auth that match:
	/// - `x-saasify-proxy-secret`: this means auth is handled by saasify, we don't
	/// care about auth anymore.
	pub fn check_header(
	) -> impl warp::Filter<Extract = (HeaderSecret,), Error = warp::Rejection> + Clone {
	let saasify_secret = get_saasify_secret();
	// See https://github.com/seanmonstar/warp/issues/503.
	let saasify_secret: &'static str = Box::leak(Box::new(saasify_secret));
	warp::header::exact_ignore_case(SAASIFY_SECRET_HEADER, saasify_secret)
	.map(|| HeaderSecret::Saasify)
	}