[DNM] ccl/changefeedccl: implement WITH diff option

Fixes cockroachdb#28666.
First commit from cockroachdb#41788.

This commit adds a `WITH diff` option to CREATE CHANGEFEED. When the
option is provided, changefeeds publications will include a `before`
field, which includes the value of the row before the CDC update was
applied.

We are able to implement this efficiently by pushing the option into
Rangefeed and performing the scan of the previous value immediately
before applying the update in the Raft processing goroutine. cockroachdb#41788
allows us to avoid performing this lookup when `WITH diff` isn't
specified for Rangefeeds, so the small (unmeasured) perf hit is
strictly opt-in.

DNM: this still needs testing and a bit of polish. It's also not clear
how we want to handle catch-up scans when the option is provided.