Backport to v16: onlineddl_vrepl flakiness and subsequent fixes

[shlomi-noach]

Description

This backport was crafted manually and includes these combined PRs, in this order:

• Internal refactor: LastError as a public struct #12321
• (re)Formalize SQLError in VReplication, add underlying wrap/unwrap functionality #12327
• Online DDL: improve retry of vreplication errors with vitess ALTER TABLE migrations #12323
• Fixing onlineddl_vrepl flakiness, and adding more tests #12325

(noteworthy that related #12318 and #12320 have already been backported, automatically)

The fixes include:

• Extracting LastError as a public struct
• Formalizing SqlError in vreplication
• Online DDL handles vreplication errors more gracefully, and retries/waits where possible
• More endtoend tests, optimize endtoend tests with new throttler config, fix wasteful wait for vreplication status

Related Issue(s)

• Online DDL: tracking issue #6926
• Bug Report: TableFilter bug in table RE listing #12317
• Bug Report: race condition between schema reads and table GC operation #12319
• Bug Report: Online DDL scheduler gives up too early on VReplication error (for vitess migrations) #12322

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

Double and triple checked the changes (since I backported manually) -- looks good.