distsql: typing information for data traveling over the wire on DistSQL streams should be statically configured

[andreimatei]

At the moment, the typing information is fed to a StreamDecoder when the first row is passed to the corresponding StreamEncoder. This leads to an awkward communication protocol where the sender/receiver need to keep track of whether they've sent/received this information.
I think it'd be a lot nicer if the typing was inferred and configured as part of the StreamEndpointSpec when the plan is created.

Jira issue: CRDB-6110

[RaduBerinde]

A note on how to add this to the planner: the schema is always available in the corresponding input sync spec; so this could easily be added in PopulateEndoints.

[dianasaur323]

moving to later to match the referenced issue #8792

[github-actions]

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[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!