schemadiff: SubsequentDiffStrategy: allow/reject multiple changes on same entity

[shlomi-noach]

Description

Some diffs between two tables, cannot be applied in MySQL in a single ALTER TABLE statement. Examples:

• Adding a column and dropping a partition.
• Adding and dropping a partition.
• Some other partitioning change scenarios.
• Adding two FULLTEXT keys.

These are MySQL limitations. Specifically for FULLTEXT, schemadiff offers a flag to tolerate multiple changes (and vitess Online DDL can then work with that).

This PR adds a more general diff hint: whether to at all allow multiple changes per table (default: allow, for backwards compatibility), or to outright reject such diff.

This can only ever apply to ALTER TABLE as no other supported change has this problem (e.g. ALTER VIEW is just one sweep atomic change anyway ; DROP TABLE obviously has nothing to apply twice, etc.)

The new hint flag will be used in followup PRs. This PR is worth being standalone.

Related Issue(s)

#15674

Checklist

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Deployment Notes

[vitess-bot]

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