schemadiff: more INSTANT algorithm considerations

[shlomi-noach]

Description

Rejecting the following scenarios:

• Reordering columns (either via MODIFY COLUMN or CHANGE COLUMN), e.g. alter table t modify column i1 int after i2
• Adding a non nullable, no-default DATETIME: alter table t add column d5 datetime(3) not null
  This one is not documented but is nonetheless rejected by MySQL - for non empty tables.
  We've found a myriad of nuances with relation to DATETIME and TIMESTAMP. The reason MySQL rejects the above is that it assumes the default is 0000-00-00 00:00:00, which it does not allow. But it does allow the same for TIMESTAMP. In both cases, if the table is empty, it is allowed. So it's about populating these columns rather than their definitions. There is further consideration for what constitutes a valid value for a DATETIME or a TIMESTAMP. e.g. '1000-01-01 00:00:00' is invalid for a TIMESTAMP as it's out of range (well before the epoch). We may work on those in a later PR.

Related Issue(s)

• Feature request: schemadiff to be aware and knowledgeable about INSTANT-capable changes #14877

Backporting

This is a bugfix, I'll backport to appropriate supported versions. After some deliberation, not going to backport this at this time.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.91%. Comparing base (e785c32) to head (cc7b217).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16678   +/-   ##
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  Coverage   68.91%   68.91%           
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  Files        1564     1564           
  Lines      201363   201371    +8     
=======================================
+ Hits       138771   138784   +13     
+ Misses      62592    62587    -5     

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