kv: add mechanism to paginate versions of a row

[ajwerner]

Is your feature request related to a problem? Please describe.

Cockroach's KV layer today only exposes versions of a key via the ExportRequest. There's a feeling more generally that the ScanRequest should be endowed with all of the powers of ExportRequest (#19749). One problem with all of this, however, is that we have no way to talk about resumption of a scan in the middle of a row. This is a major impediment to proper memory management for such operations.

Describe the solution you'd like
We'd need some new API parameter which allows requests and resume spans to indicate timestamp bounds.

Jira issue: CRDB-2980

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