Add warp::auth::basic() filter

[586837r]

You use it like this:

use warp::Filter;

let auth = warp::auth::basic()
    .realm("access")
    .allow("user", "1234");

let route = warp::any()
    .map(warp::reply)
    .with(auth);

#751 also implements basic authentication.

Contrary to theirs, mine:

• is very simple and limited
• returns an impl Reply instead of a rejection on authorization failure
• uses an immutable set of authorizations
• realm is optional

[mike-kfed]

I like the implementation using builder pattern and usage as a with() filter.

Only objections are

• the user/password storage is not encrypted, therefore insecure (also not sure this should be part of warp, something that can for example authenticate against htpasswd files could/should be an extra crate)
• no way to use a custom lookup function for e.g. a user/pass database (or when used for Bearer auth to verify tokens)

[gralpli]

Just an additional thought: the password comparison is not constant-time and might therefore be vulnerable to timing attacks.