schemadiff: formalize SchemaDiff as the only schema diffing mechanism

[shlomi-noach]

Description

Followup to #12551
This PR hides the original Schema.Diff() function as Schema.diff(), leaving SchemaDiff() as the only mechanism for diffing two schemas.

The resulting SchemaDiff object offers these two public functions for getting the diffs:

• OrderedDiffs(): typically this is what you want to use; a linearized slice of diffs, which are guaranteed to be conflict free and keep the schema valid at each step (a non-nil error is returned if it is impossible to get a conflict free list)
• UnorderedDiffs(): a list of diffs - same diffs as above - but in non guaranteed and non-tested order. The order will work in the vast majority of cases and uses some basic heuristics. The use case for this function is for when the above OrderedDiffs() fails and one still wants to put their hands on a list of diffs.

Related Issue(s)

• schemadiff: diffing two schemas generates a rich SchemaDiff object #12551
• schemadiff: tracking issue #10203

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

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