plan/executor: handle optimizer hint properly for cartesian join

[eurekaka]

What problem does this PR solve?

Fix #9032

What is changed and how it works?

• allow merge join executor to handle cartesian product;
• raise warning and choose other join implementations if TIDB_INLJ is specified;

Check List

Tests

• Unit test

Code changes

N/A

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

[eurekaka]

/run-all-tests

[zz-jason]

LGTM

[eurekaka]

/run-unit-test

[eurekaka]

Comments addressed, @XuHuaiyu PTAL

[codecov-io]

Codecov Report

Merging #9037 into master will decrease coverage by 0.02%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #9037      +/-   ##
==========================================
- Coverage   67.28%   67.26%   -0.03%     
==========================================
  Files         371      371              
  Lines       75930    75920      -10     
==========================================
- Hits        51092    51070      -22     
- Misses      20332    20346      +14     
+ Partials     4506     4504       -2

Impacted Files	Coverage Δ
executor/merge_join.go	81.5% <0%> (ø)	⬆️
planner/core/exhaust_physical_plans.go	92.1% <100%> (+0.03%)	⬆️
ddl/delete_range.go	74.28% <0%> (-4.58%)	⬇️
planner/core/planbuilder.go	49.07% <0%> (-1.05%)	⬇️
executor/index_lookup_join.go	77.28% <0%> (-0.64%)	⬇️
executor/executor.go	66.66% <0%> (-0.15%)	⬇️
planner/core/logical_plan_builder.go	74.98% <0%> (+0.25%)	⬆️
executor/join.go	78.44% <0%> (+0.51%)	⬆️

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[XuHuaiyu]

LGTM