Online DDL/VReplication: support non-UTF8 character sets

[shlomi-noach]

Description

Supporting non UTF8 character sets in OnlineDDL/VReplication:

• Table can be created in any character set
• Online DDL supports copying table data even if character set is non utf8
• Supports converting from utf8 to non-utf8
• Supports converting from non-utf8 to utf8
• Supports oncverting from non-utf8 to non-utf8 (e.g. latin1 to latin2)
• Limited set of character sets supports (but the major ones are supported)

Implementation:

• VReplication's filter->rule now has a new field, CharsetConversion, which tells Vreplication:

  • for every textual column
  • which has seen a change in character set
  • what was the original charset and what is the new charset
    VReplication plan will programmatically convert binlog text values according to the above.

• VCopier adds a convert(<column> using utf8mb4) to any textual column where charset/collation has changed

• VStreamer identifies the convert() function and similarly runs a convert(... using utf8mb4) on same column as VCopier hinted, so that we convert the columns on the source side

Related Issue(s)

• Tracking: import gh-ost test suite to Vitess/VReplication #8179
• Towards a VReplication/OnlineDDL testing suite #8181

Checklist

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

Notes

Submitting code changes with a lot of commented code and with debug info, as draft pull request. Will clean it up before making it ready for review.

[shlomi-noach]

Ready for review!

Notable:

• online DDL instructs vstreamer to read all textual columns with convert(? using utf8mb4)
• online DDL tells vreplication/rule whether a column has a from-to charset conversion
• binary logs text values are always in utf8, so this lines up both
• replication plan examines textual column and sees if there's a charset conversion, and decodes programmatically as needed

Tests are good and present multiple interesting cases (from/to utf8, from/to latin1, latin1-to-latin2, nullable, ...)

[shlomi-noach]

Tests added in this PR:

• alter-charset-non-utf8-80
• alter-charset-non-utf8
• convert-utf8mb4

[shlomi-noach]

Resolved conflicts having merged #8275, and cleaned up the code a bit.

[rohit-nayak-ps]

nice! lgtm