beacon: Add a minimal gRPC service

This moves the following calls to the new beacon gRPC service:
 * `Consensus.GetEpoch`
 * `Consensus.WaitEpoch`
 * `Consensus.BeaconConsensusParameters` -> `Beacon.ConsensusParameters`

.changelog/3666.internal.md

@@ -0,0 +1,4 @@
+beacon: Add a minimal gRPC service
+
+I would put something more descriptive in here, but the change log
+fragment CI linter continues to terrorize me.

go/beacon/api/api.go

@@ -58,19 +58,26 @@ type Backend interface {
 	// epoch.
 	GetEpochBlock(context.Context, EpochTime) (int64, error)
 
+	// WaitEpoch waits for a specific epoch.
+	//
+	// Note that an epoch is considered reached even if any epoch greater
+	// than the one specified is reached (e.g., that the current epoch
+	// is already in the future).
+	WaitEpoch(ctx context.Context, epoch EpochTime) error
+
 	// WatchEpochs returns a channel that produces a stream of messages
 	// on epoch transitions.
 	//
 	// Upon subscription the current epoch is sent immediately.
-	WatchEpochs() (<-chan EpochTime, *pubsub.Subscription)
+	WatchEpochs(ctx context.Context) (<-chan EpochTime, pubsub.ClosableSubscription, error)
 
 	// WatchLatestEpoch returns a channel that produces a stream of
 	// messages on epoch transitions. If an epoch transition happens
 	// before the previous epoch is read from the channel, the old
 	// epochs are overwritten.
 	//
 	// Upon subscription the current epoch is sent immediately.
-	WatchLatestEpoch() (<-chan EpochTime, *pubsub.Subscription)
+	WatchLatestEpoch(ctx context.Context) (<-chan EpochTime, pubsub.ClosableSubscription, error)
 
 	// GetBeacon gets the beacon for the provided block height.
 	// Calling this method with height `consensus.HeightLatest` should