kv: audit uses of (roachpb.Value).RawBytes

**Describe the problem**

In #131348 we have added an option that allows senders of GetRequest and ScanRequest to request. In the future we may consider making this option the default, but before that is possible, we need to be sure that existing users of the RawBytes aren't interacting with that fields in a way incompatible with the extended encoding. This includes:

- Callers checking `len(value.RawBytes) == 0` or `value.RawBytes == nil` to determine if a value is tombstone.
- Callers attempting to manually parse RawBytes without the aid of the helper methods on `roachpb.Value`.
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Jira issue: CRDB-42646