OnlineDDL to use schemadiff version capabilities; refactor some flavor code.

[shlomi-noach]

Description

Followup to #14878, which introduced schemadiff logic for detecting INSTANT-able changes. In this PR:

• onlineddl/analysis.go uses said schemadiff analysis, removing its own overlapping logic.
• Improves the convenience of getting and using a CapableOf function.
  • Now, all it takes is a server version, and you get a function. See mysql.ServerVersionCapableOf(version) convenience function. I've switched all code that gets a CapableOf to use this new accessor function.
  • Refactored flavor such that all flavors contain a versionString field. It is then used by their supportsCapability(...) implementations, rather than having a version string supplied externally as a function argument.

Note: none of the Online DDL tests changed in essence - the point of this refactor is to preserve behavior through new implementation. The existing tests have good coverage and if they pass that means the refactor is good.

Related Issue(s)

Closes #14877

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[mattlord]

LGTM! I just had the minor questions/comments/suggestions. I'll leave it to you to resolve as you feel is best.

[mattlord]

This now assumes that the serverVersion was specified otherwise. That's fine, I think, but we should probably add some error handling / unit test code covering the case where it's not set.

[shlomi-noach]

Added unit test as suggested. In actual connections, serverVersion is always non-empty; the existing logic for setting a connection's flavor is to return nil flavor when there is no known flavor for the given server version (whether server version is empty, or something ridiculous as "1792.3343.3"). Just pointing out that on this respect there's nothing new this PR adds.

[mattlord]

I'm guessing that if f.serverVersion is the zeroval, then this will just always return false, nil., which seems fine to me. We should add a unit test case for that though.

[shlomi-noach]

Added.

[mattlord]

I think we should have test coverage for what the expected behavior is if there's a future use of the flavor as we did before, i.e. with no serverVersion specified like it was here.

[shlomi-noach]

Added.

[mattlord]

Here, I think we could just add:

		{
			version:    "",
			capability: capabilities.CheckConstraintsCapability,
			isCapable:  false,
		},

No?

[shlomi-noach]

Added.

[shlomi-noach]

There will actually be a followup PR, I just don't want to overload this PR. I'm unhappy with the amount of information returned by CapableOfInstantDDL(). It only returns a bool, without indicating which migrations were at all eligible for INSTANT, or, which migrations were impossible for INSTANT (e.g. ALTER TABLE ... ADD INDEX), or, which migrations were impartial/irrelevant to INSTANT (e.g. a CREATE TABLE).

[shlomi-noach]

Followup PR in #14900, will only be taken out of Draft when this PR is merged.