Support receiver targeted silences

Proposal

Requires #4951

It's extremely difficult constrain users' ability to create silences in Alertmanager. Because silences match alerts with matchers, users need to understand the mapping of labels to alerts to receivers in order to target their silences. For wide-reaching silences, it's extremely easy to silence more alerts than expected.

Because Alertmanager is intended for somewhat multi-tenant cases, it makes sense to provide a mechanism for users to target silences in terms of receiver instead of just matchers. This would allow for silences that only silence alerts for a user's receivers instead of for everybody's receivers.

We could implement this via a new receiver_matchers field on the silence which allows a silence to target only matching receivers. #4951 is realistically required for this because it will allow silences to apply in some groups but not others.

This would be massively useful because it would allow users to think of silences in terms of just their receivers instead of the cross-team impact. It decouples the concern of "what to silence" from "who to silence it for".