*WIP* schemadiff: Diff() of schemas ordered by valid dependencies

[shlomi-noach]

Description

A schemadiff's Schema is normalized to have tables in foreign key dependency order (parents come first, children later), as well as in view dependency order (tables first, then views that only rely on tables, then 2nd level views etc.). This makes it possible to create, or apply, the schema by iterating its entities in order.

With this PR, we also sort the Diff() of two schemas, such that the diff can be applied in-order to a schema and without error. At this time, the diff is sorted like so:

• CREATE TABLE follow the same ordering for schema creation, ie. foreign key parents come first, then their children, and recursively onwards.
• CREATE TABLE comes before CREATE VIEW, again following same logic as illustrated above.
• DROP VIEW uses reverse order of creation, such that we first drop the views with most dependencies, then those with fewest dependencies
• DROP TABLE uses reverse order of creation, such that we first DROP a foreign key relationship's child before DROPping its parent.

There is still work to be done where it comes to modifying foreign key constraints in existing tables.

Related Issue(s)

• Foreign keys tracking issue: Experimental foreign keys tracking issue #11975
• schemadiff tracking issue: schemadiff: tracking issue #10203
• schemadiff: validate and apply foreign key indexes #12026

Checklist

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

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a test is added or modified, there should be a documentation on top of the test to explain what the expected behavior is what the test does.

If a new flag is being introduced:

• Is it really necessary to add this flag?
• Flag names should be clear and intuitive (as far as possible)
• Help text should be descriptive.
• Flag names should use dashes (-) as word separators rather than underscores (_).

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow should be required, the maintainer team should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should include a link to an issue that describes the bug.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from VTop, if used there.

[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.

[shlomi-noach]

Superseded by #12551