req-resp/litep2p: Reject inbound requests from banned peers (#7158)

This PR rejects inbound requests from banned peers (reputation is below
the banned threshold).

This mirrors the request-response implementation from the libp2p side.
I won't expect this to get triggered too often, but we'll monitor this
metric.

While at it, have registered a new inbound failure metric to have
visibility into this.

Discovered during the investigation of:
https://github.com/paritytech/polkadot-sdk/issues/7076#issuecomment-2589613046

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>