Touches cockroachdb#67562.

This commit introduces a new RoutingPolicy configuration that lives on a
BatchRequest header. A request's routing policy specifies how the
request should be routed to the replicas of its target range(s) by the
DistSender. There are initially two routing policies:
```
enum RoutingPolicy {
  // LEASEHOLDER means that the DistSender should route the request to the
  // leaseholder replica(s) of its target range(s).
  LEASEHOLDER = 0;
  // NEAREST means that the DistSender should route the request to the
  // nearest replica(s) of its target range(s).
  NEAREST = 1;
}
```

The default policy is `LEASEHOLDER`.

Routing policies allow us to stop overloading the use of the
ReadConsistency enum to dictate both how the client should route a
request to a server and which kinds of requests should be eligible to be
served by a given replica. Routing policies are a client-side only
concept. They do not dictate which replicas in a range are eligible to
serve the request, only which replicas are considered as targets by the
DistSender, and in which order. A request that is routed to an
ineligible replica (a function of request type, timestamp, and read
consistency) will be rejected by that replica and the DistSender will
target another replica in the range.

As discussed in cockroachdb#67725 (review),
we will likely need to introduce a third routing policy that called
`SINGLE_REPLICA` to address cockroachdb#67554. This policy would be accompanied by
a ReplicaDescriptor and would specify that a given request must be sent
to that replica and DistSender should throw an error if the replica is
not part of its cached range descriptor. This is important to ensure
that a QueryResolvedTimestampRequest and its follow-up ScanRequest are
both sent to the same replica.