kvflowcontrol: add raft, scheduler, ready interface

[kvoli]

Add three new interfaces, which define methods the replication AC v2 controller will call or use, eventually implemented ontop of raft:

type RaftEvent interface {
	// Ready can return nil if there is no Ready.
	Ready() Ready
}

type Ready interface {
	// GetEntries represents the new entries that are being added to the log.
	// This may be empty.
	GetEntries() []raftpb.Entry
}

// RaftInterface abstracts what the RangeController needs from the raft
// package, when running at the leader.
type RaftInterface interface {
	// FollowerState returns the current state of a follower. The value of
	// Match, Next, Admitted are populated iff in StateReplicate. All entries >=
	// Next have not had MsgApps constructed during the lifetime of this
	// StateReplicate (they may have been constructed previously).
	FollowerState(replicaID roachpb.ReplicaID) FollowerStateInfo
	// FollowerStateRLocked return the current state of a follower.
	FollowerStateRLocked(replicaID roachpb.ReplicaID) FollowerStateInfo
	// NextUnstableIndex returns the index of the next entry that will be sent to
	// local storage, if there are any. All entries < this index are either stored,
	// or have been sent to storage.
	NextUnstableIndex() uint64
	// MakeMsgApp is used to construct a MsgApp for entries in [start, end).
	MakeMsgApp(replicaID roachpb.ReplicaID, start, end uint64, maxSize int64) (raftpb.Message, error)
}

// Scheduler abstracts the raftScheduler to allow the RangeController to
// schedule its own internal processing. This internal processing is to pop
// some entries from the send queue and send them in a MsgApp.
type Scheduler interface {
	ScheduleControllerEvent(rangeID roachpb.RangeID)
}

See https://github.com/sumeerbhola/cockroach/blob/2064b279e95bbd68b0a7898503168d384cd0158c/pkg/kv/kvserver/kvflowcontrol/kvflowconnectedstream/stream.go#L653

Jira issue: CRDB-40754

Epic CRDB-37515