WIP: refactoring Online DDL "immediate operations" code

[shlomi-noach]

Description

This is an ongoing experiment in refactoring the immediate operations code, where Online DDL executor can/might invoke an ALGORITHM=INSTANT migration. The objectives for this experiment are:

• Improved testing
• Improved visibility (adding information in schema_migrations)
• Potentially extracting this logic outside go/vt/vttablet/onlineddl and into go/vt/schemadiff

Related Issue(s)

#11910

Checklist

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[vitess-bot]

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[shlomi-noach]

Some work extracted to #14878

[shlomi-noach]

further changes in #14883, it is likely that this PR will be closed without merging soon.

[shlomi-noach]

Closing this, given alternative approach in #14883.