DDL strategy flag --unsafe-allow-foreign-keys implies setting FOREIGN_KEY_CHECKS=0

[shlomi-noach]

Description

Applying in both direct as well as online strategies, adding --unsafe-allow-foreign-keys DDL strategy flag implies a SET FOREIGN_KEY_CHECKS=0 before executing the SQL.

With this, the user is allowed to create cyclic foreign key relationship, as in:

create table t11 (id int primary key, i int, constraint f11 foreign key (i) references t12 (id));
create table t12 (id int primary key, i int, constraint f12 foreign key (i) references t11 (id));

Note: this changes existing behavior. However, since existing behavior is experimental, as clearly indicated by --unsafe- prefix, we are good to make that change.

Related Issue(s)

Addresses #15430

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 18.75000% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 65.73%. Comparing base (6c73053) to head (3c3c4a0).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	25.00%	9 Missing ⚠️
go/vt/schemamanager/tablet_executor.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15432   +/-   ##
=======================================
  Coverage   65.72%   65.73%           
=======================================
  Files        1563     1562    -1     
  Lines      194027   193975   -52     
=======================================
- Hits       127529   127501   -28     
+ Misses      66498    66474   -24     

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