v18 backport: Online DDL: avoid SQL's CONVERT(...), convert programmatically if needed

[shlomi-noach]

Manual backport of #16597 to release-18.0

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Description

Fixes #16023

We have a clear picture and a fix to #16023. The original reason why we needed convert() in the first place is that vreplication and vstreamer both issue a SET NAMES binary. We will want to change that in the future, but this PR in the meantime confirms to the binary connection charset.

So we used convert() to turn textual values into utf8mb4. On the other side, vplayer is reading events from the binary log. It used programmatic conversion (charset.Convert()) of the data to utf8mb4 to align with vcopier.

What we are doing now:

• We do not use convert(), solving the sorting issue described in Bug Report: OnlineDDL PK conversion results in table scans #16023 (comment)
• For vcopier read data, We do introduce programmatic conversion of non-utf columns into their designated charsets.
• For vplayer, we do not convert at all if both source and target have same charset
• For vplayer, we do apply programmatic conversion of non-utf columns into their designated charsets, in a similar logic as for vcopier.
• If there's a charset.Convert() error, we translate it into ERROR 1366 ("Incorrect string value ..."), which is a terminal error in vreplication, and so the migration bails out as soon as that happens. This can happen if e.g. we're converting a UTF column into ASCII and the UTF column contains a smiley emoji.

Because we do not convert the original charset to utf8mb4, we get to programmatically convert it to the specific target column. Previously (and this is perhaps the last piece of magic I have not digged into yet, and again likely to be caused by the binary charset) we did not need to convert into the target charset.

All the tests remain the same, and we introduce a couple new ones.

Related Issue(s)

• Bug Report: OnlineDDL PK conversion results in table scans #16023
• supersedes Online DDL: do not CONVERT column charset when migration does not modify column's charset or collation #16594

Backport

I wish to backport this to all supported versions, seeing that this is a bugfix: without this fix some migrations will slow down to a near halt.

Checklist

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

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