kvflowcontrol: add store streams token counter interface

[kvoli]

Define a new interface in kvflowcontrol which supports a similar interface to:

// StoreStreamsTokenCounter is one per node.
type StoreStreamsTokenCounter interface {
	// EvalTokenCounterForStrea returns the eval token counter for the given
	// stream.
	EvalTokenCounterForStream(kvflowcontrol.Stream) TokenCounter
	// SendTokenCounterForStream returns the send token counter for the given
	// stream.
	SendTokenCounterForStream(kvflowcontrol.Stream) TokenCounter
	// GetEvalTokenCounters returns every eval token counter. This is used for
	// metrics and inspection.
	GetEvalTokenCounters() map[kvflowcontrol.Stream]TokenCounter
	// GetSendTokenCounters returns every send token counter. This is used for
	// metrics and inspection.
	GetSendTokenCounters() map[kvflowcontrol.Stream]TokenCounter
}

Also see the relevant prototype code.

Jira issue: CRDB-40736

Epic CRDB-37515