Add RevokeAuthorizations func to the SA gRPC service.

[ezekiel]

No description provided.

[aarongable]

Thinking out loud:

The reason that DeactivateAuthorization operates on a single authz (takes a single authzID as its input) is that this code path is initiated by the subscriber. The ACME API exposes a way to deactivate one authz at a time, so the code path all the way through the RA and SA does the same.

But revoking authzs will likely be a bulk operation. We only have two scenarios in which we want to revoke authzs:

• During an incident, to prevent further issuance via reuse of previous (presumably non-compliant) authorizations. This would either be a bulk revocation of all authzs prior to a specific time, or a bulk revocation of all authzs from a large list of IDs extracted from logs.
• During revocation of a cert, as this PR is building towards. In this case, we're revoking all authzs for a specific regID+identifier pair.

So maybe it makes more sense for this to be

rpc RevokeAuthorizationsFor(RevokeAuthorizationsForRequest) returns (google.protobuf.Empty) {}

message RevokeAuthorizationsForRequest {
  int64 registrationID = 1;
  core.Identifier identifier = 2;
}

[ezekiel]

Very helpful thoughts, thank you!