kv: expose byte limits through kv.Txn API

Currently, the kv.Txn API exposes key limits through the maxRows arg (which is poorly named) of its Scan and ReverseScan methods, but it does not expose an ability to impose a byte limit. Byte limits were introduced into the kv proto API in https://github.com/cockroachdb/cockroach/commit/417aa0843dbc1b816d7326cf223e141aa817bd30. However, because of this limitation in the kv.Txn API, users who want byte limits need to construct raw roachpb protos to be able to use them.

We should add byte limits to the kv.Txn and kv.DB API.

Jira issue: CRDB-6709