schemadiff: identify a FK sequential execution scenario, and more

[shlomi-noach]

Description

In this PR we programmatically identify the following scenario:

• We diff two schemas
• We identify one table (child) adds a new foreign key (pointing to a parent).
• And we identify the parent table adds an appropriate index on the FK referenced columns.
• And we identify there isn't an already existing index on referenced columns.

In this scenario, the two changes are strictly sequential, and the two migrations will never be able to run concurrently.

UPDATE: we've identified new scenarios, added in this PR. See followup comments.

Related Issue(s)

• Addendum to schemadiff: diffing two schemas generates a rich SchemaDiff object #12551
• Experimental foreign keys tracking issue #11975

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
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• Documentation was added or is not required

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[vitess-bot]

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[shlomi-noach]

All right, in the couple hours since opening this PR I've found a whole new family of FK-related dependencies. Basically, any CREATE TABLE that has a FOREIGN KEY constraint, introduces a foreign key in the same way that ALTER TABLE ... ADD FOREIGN KEY does. And while CREATE TABLE is an immediate operation, some scenarios still require strict sequential execution. Updated the code and the tests.

[notfelineit]

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