dataflow: add dataflow rendering to EXPLAIN plans

[maddyblue]

If there's a view with an index on a column, a point select on that column will be significantly faster. But an EXPLAIN will not surface the use of the index. We should add this information to EXPLAIN. In this case looking at the rendered dataflow will show index instead of scan. Users will be able to self-serve improve their queries better with this information.

Update: 2022/02/08

We added a rudimentary version of this in #8515. We were holding off documenting it, because it's very verbose and we expected to follow-up sooner. We didn't follow-up, it's still verbose, but came in handy a few times already, so I'm proposing the following sub-tasks:

• Rename explain physical plan to explain dataflow plan
• Add docs under docs/sql/explain following the existing structure (sections "Reading ...", "Operators in", and an extra warning that it's experimental)

There were various discussions in the pats to take a holistic look at explain output, but we didn't find a proper owner (cc @JLDLaughlin @andrioni). I think it's fine to do the above minimal things even if we expect things to change in the future.

[uce]

This is mostly ready on the dataflow side but requires adding the SQL commands and pretty printing (we probably need to discuss what we actually want in there).

[uce]

The initial work has been done, but we'll keep this open to scope out the follow-up tasks.

[aalexandrov]

@uce: can you put labels to the areas where the outstanding tasks fall? I've tentatively added A-optimization here but I'm not sure whether there is something more to be done there.

[aalexandrov]

A variant of this was done in #13137.