release-20.2: opt,sql: add EXPLAIN (OPT, MEMO) to show optimizer memo

[rytaft]

Backport 1/1 commits from #67702.

/cc @cockroachdb/release

Release justification: This adds a new debugging tool that we can use to help understand issues in the 20.2 release branch. It is a minimal change to EXPLAIN functionality.

I will wait two weeks before merging.

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Release note (sql change): Added a new EXPLAIN flag, MEMO, to be used
with EXPLAIN (OPT). When the MEMO flag is passed, a representation of the
optimizer memo will be printed along with the best plan. The MEMO flag
can be used in combination with other flags such as CATALOG and VERBOSE.
For example, EXPLAIN (OPT, MEMO, VERBOSE) will print the memo along with
verbose output for the best plan.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[mgartner]

Reviewed 12 of 12 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @RaduBerinde)

[rytaft]

TFTR!