schemadiff: improved diff ordering with various foreign key strategies

[shlomi-noach]

Description

We introduce a new foreign key diff strategy flag and improve the diff sorting mechanism. The strategy flags are now: (from strict to loose):

• ForeignKeyCheckStrategyStrict
• ForeignKeyCheckStrategyCreateTableFirst
• ForeignKeyCheckStrategyIgnore

The main change in this PR is the understanding that "stricter is better, if possible". In this PR, if we request a diff using a loose ForeignKeyCheckStrategyIgnore strategy, and the diff can be fulfilled by the stricter ForeignKeyCheckStrategyStrict, then it will. If not, and can be fulfilled by still stricter ForeignKeyCheckStrategyCreateTableFirst, it will.

The converse is not true: if we request a diff using ForeignKeyCheckStrategyStrict strategy, then it may only be fulfilled via ForeignKeyCheckStrategyStrict.

In effect, this is only relevant when cyclic foreign keys are introduced. When we say "cyclic", we refer to tables referencing each other such as t1->t2->t3->t1. This PR is not about existence of cyclic tables; it is about introducing a cyclic change such that the diff themselves might depend on each other. And the discussion is: "what is a valid order of diffs, such that we can introduce one diff at a time, maintain a valid schema, and end of with the desired schema"?

It is not always possible to get such ordering given a set of changes, and the different strategies tell us how to tackle such situations:

• ForeignKeyCheckStrategyStrict: there absolutely must be a valid sequence, or else we return an error.
• ForeignKeyCheckStrategyCreateTableFirst: it's possible to have an invalid sequence, but we tolerate a sequence where the first change is ALTER TABLE, even if it introduces a dependency which is not yet met. The rest of changes must then all line up. At the end of all changes, we must achieve a valid schema.
• ForeignKeyCheckStrategyIgnore: we don't care at all if there's no valid sequence. But at the end of all changes, we must achieve a valid schema.

Related Issue(s)

#15430

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.19%. Comparing base (87ecae7) to head (0da285a).

Files	Patch %	Lines
go/vt/schemadiff/schema_diff.go	94.44%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16081      +/-   ##
==========================================
- Coverage   68.19%   68.19%   -0.01%     
==========================================
  Files        1541     1541              
  Lines      197369   197391      +22     
==========================================
+ Hits       134605   134618      +13     
- Misses      62764    62773       +9     

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