[explainability] change EXPLAIN ... AS TEXT in tests to EXPLAIN ... AS VERBOSE TEXT ...

[mgree]

We need to keep the current verbose EXPLAIN output for our testing, but we'll want to change the default output to something else.

This PR creates an ExplainFormat::VerboseText, parsed as VERBOSE TEXT, which behaves identically to ExplainFormat::Text, parsed as TEXT (the default). All tests are changed to use VERBOSE TEXT explicitly. Later, we can change what TEXT does without breaking our testing.

Motivation

• This PR adds a known-desirable feature.
  https://github.com/MaterializeInc/database-issues/issues/8889

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.