Make Foreign_key_checks a Vitess Aware variable

[GuptaManan100]

Description

This PR makes FOREIGN_KEY_CHECKS as a Vitess aware variable. Planning gets affected because of this variable, so introducing Vitess managed foreign keys prompted this change. We now keep track of this variable in the session. This PR also adds the capability to the sqlparser to parse and read the optimiser hint comments to find the value of FOREIGN_KEY_CHECKS if specified as part of a SET_VAR optimiser hint.

Once this information has been processed, we store it in the vcursor and use this for planning. The plan hash has also been changed to take into account the foreign key checks state because the plans produced in different states are different and need to be cached separately.

Plan tests have been added and fuzzer changes have been made to test out this new functionality.

Related Issue(s)

• RFC: Foreign Key Support in Vitess #12967

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[vmg]

Good change Manan! Ended up being much smaller than I expected ✨

[GuptaManan100]

Thank-you very much! 💕