Online DDL declarative migrations now use schemadiff, removing tengo

[shlomi-noach]

Description

Now that #9719 is merged, we switch -declarative migrations from using tengo into using schemadiff. In this change:

• executor.go now uses schemadiff's declarative diff based on SHOW CREATE TABLE output from original and ghost tables
• There is one less table to create in the interim of the process
• We utilize existing Vitess DB connections, and remove use of sqlx
• No need to create a temporary account
• No longer using tengo library
• Removed some boilerplate parsing code
• Removed many package dependencies (see go.mod)

Tests-wise, the PR remains covered by onlineddl_declarative.

Related Issue(s)

• Introducing schemadiff, a declarative diff for table/view CREATE statements #9719
• Online DDL: tracking issue #6926

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

[vmg]

This is, again, great just from the sheer amount of dependencies it removes. I don't have a good enough understanding of the subsystem to approve the changes so I'm basically giving my thumbs up to the go.mod diff. :)

[deepthi]

/assign @vitessio/query-serving

[shlomi-noach]

call for review 🙏

[shlomi-noach]

Tracking issue: #10203