Currently, the proxy-injector may set the following environment
variables to configure the policies that the proxy should use for
specific inbound ports:

- `LINKERD2_PROXY_INBOUND_PORTS_DISABLE_PROTOCOL_DETECTION`
- `LINKERD2_PROXY_INBOUND_PORTS_REQUIRE_IDENTITY`
- `LINKERD2_PROXY_INBOUND_PORTS_REQUIRE_TLS`

However, when the proxy is configured with a policy service address,
these environment variables are never used, even when they are set.
Instead, the proxy will always look up those ports with the policy
controller, which will then tell it to do...what the environment
variable would have already told it to do (because the env vars are
generated based on the same annotations that would tell the policy
controller what policy to send). This means we do unnecessary policy
resolutions for those ports.

This branch updates the proxy so that these environment variables are
always honored, even when a policy controller is configured. This way,
the unnecessary lookups are avoided. Critically, we should note that we
will never do a policy lookup for a port that is configured by one of
these environment variables.

If a port is present in both `LINKERD2_PROXY_INBOUND_PORTS` (the list of
ports for which we proactively start policy controller watches), as well
as configured by one of the policy env vars, we will still look that
port up with the policy controller, rather than using the env var
configuration. A warning is logged in this case.