fix(injector): improved support for native sidecar servers

(Extracted from #14566)

This improves the injector logic by accounting for ports exposed in native sidecar containers:

- when consuming the `config.linkerd.io/opaque-ports` annotations
  pointing to (named or integer) ports in init containers
- when populating the proxy's `LINKERD2_PROXY_INBOUND_PORTS` env var

The `webhook_tests.go` have been expanded to test injection to a pod
with a memcached native sidecar container (never mind the contrivedness
of the example).

Author: alpeb