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