shcemadiff: support INSTANT DDL for changing column visibility

[shlomi-noach]

Description

While not documented in https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html, changing column visibility is INSTANT-able, and this PR adds this analysis in schemadiff.

While at it, INSTANT DDL analysis now also supports CHANGE COLUMN statements where the column name is unchanged, as well as ALTER COLUMN statements. It's noteworthy that schemadiff itself only generates MODIFY COLUMN statements and never either of these two, and this support comes just for completeness.

Related Issue(s)

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

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Project coverage is 68.64%. Comparing base (fab9071) to head (316eb02).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/schemadiff/capability.go	91.66%	3 Missing ⚠️
go/mysql/capabilities/capability.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16503      +/-   ##
==========================================
+ Coverage   68.63%   68.64%   +0.01%     
==========================================
  Files        1551     1552       +1     
  Lines      199515   199559      +44     
==========================================
+ Hits       136938   136990      +52     
+ Misses      62577    62569       -8     

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[mattlord]

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