Online DDL/Vreplication suite: support ENUM->VARCHAR/TEXT type change

[shlomi-noach]

Description

This is a difficult one to solve and this PR is not ready yet. solved.

See github/gh-ost#642 for issue description in gh-ost and see openark/gh-ost#18 for gh-ost solution.

When the user converts a column's type from from ENUM to VARCHAR they expect the new column to contain the textual representation of the enum value. And that's what happens for rows copied via VCopier. But VPlayer copies the numeric value.

Problem is VPlayer (or table plan builder) do not have the information about what the enum looks like. WIP.

Related Issue(s)

#8181

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[shlomi-noach]

This is now ready to review. Noteworthy:

• converting ENUM to TEXT requires changes to VReplication logic. This is because VReplication needs to know how to interpret an enum valu in the binary logs.
• OnlineDDL/vrepl is responsible for identifying an Online DDL which converts an enum to varchar/text
• The, it parses the enum definition
• And stores it in VReplication's Rule: a mapping between column name to its enum values. The existence of the mapping will indicate to VReplication that it must use the mapping.
• To that effect, I've added enum_text in rule's proto. This has to happen, because VReplicatio nitself needs to be able to tell what to do with an enum value.
• We go the way down to TablePlan (replicator_plan.go). In applyChange, TablePlan looks at the field it read from the binary log. If it is an enum and is non-NULL and there is a mapping for this column, then instead of binding the enum's numeric value, it binds the textual value.
• This PR does not change behavior for enums otherwise. e.g. if there's a table with enum and there's Online DDL or MoveTables or whatever, without modifying the enum, then behavior is unaffected by this PR.

[shlomi-noach]

Hmmm, I just added a new complex test: enum-to-varchar-rename which both converts a column from enum to test and renames it. This fails. Looking into.

[shlomi-noach]

Converting back to draft while checking the new test

[shlomi-noach]

Ready for review, final.

Fixed the issue failing the added test.
Also, refactored code to match changes in another PR, #8322. Both PRs touch replicator plan in similar way.
Whiever PR gets merged first, will cause merge conflicts on the other, but now they're much more aligned.

[rohit-nayak-ps]

lgtm.