Add rate-limiters to ServerPolicy

[alpeb]

This adds the local_rate_limit module to the server-policy crate, that
ServerPolicy uses for its new local_rate_limit field, containing
three optional rate-limiters: total, identity, overrides (this one is
really a vector of limiters, one per configured override).

I tried putting that under Protocol instead, but the PartialEq
requirement made it very hard to follow. Server OTOH doesn't really
require that trait, so I was able to remove it and accommodate the
limiters.

I made sure to avoid pulling the dashmap dependency in governor; I
haven't checked yet the necessity of the "jitter" and "quanta" features.

This temporarily overrides linkerd2-proxy-api dependency to pick changes
from linkerd/linkerd2-proxy-api#388

Update

The HttpPolicyService middleware has been expanded to call the local_rate_limit module above to perform the rate-limit check.

[olix0r]

nit: slightly more readable:

Suggested change

	match err {
	RateLimitError::PerIdentity(rps) => assert_eq!(rps, &std::num::NonZeroU32::new(1).unwrap()),
	_ => panic!("unexpected error"),
	};
	match err {
	RateLimitError::PerIdentity(rps) => assert_eq!(rps.into(), 1),
	_ => panic!("unexpected error"),
	}

[alpeb]

Clippy doesn't like that 😞

error[E0283]: type annotations needed
   --> linkerd/app/inbound/src/policy/http/tests.rs:465:60
    |
465 |         RateLimitError::PerIdentity(rps) => assert_eq!(rps.into(), 1),
    |                                                            ^^^^
    |
    = note: cannot satisfy `_: std::convert::From<&std::num::NonZeroU32>`
    = note: required for `&std::num::NonZeroU32` to implement `std::convert::Into<_>`
help: try using a fully qualified path to specify the expected types
    |
465 |         RateLimitError::PerIdentity(rps) => assert_eq!(<&std::num::NonZeroU32 as std::convert::Into<T>>::into(rps), 1),
    |                                                        +++++++++++++++++++++++++++++++++++++++++++++++++++++++   ~

[olix0r]

Take note that we'll want to update the commit message to reflect the current state of this branch.