sql: add support for IF NOT EXISTS for ALTER TABLE ADD CONSTRAINT

[ekalinin]

Fixes #53007

This commit adds the option IF NOT EXISTS to the ALTER TABLE ADD CONSTRAINT. With this option pgcode.DuplicateObject will not be raised.

Release note (sql change): Add support for IF NOT EXISTS option for
ALTER TABLE ADD CONSTRAINT command.

[blathers-crl]

Thank you for contributing to CockroachDB. Please ensure you have followed the guidelines for creating a PR.

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I have added a few people who may be able to assist in reviewing:

• @glennfawcett (author of Add syntax for to use "IF NOT EXISTS" modifier to "ALTER TABLE add CONSTRAINT" #53007)
• @awoods187 (commented on Add syntax for to use "IF NOT EXISTS" modifier to "ALTER TABLE add CONSTRAINT" #53007)
• @rohany (commented on Add syntax for to use "IF NOT EXISTS" modifier to "ALTER TABLE add CONSTRAINT" #53007)

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[cockroach-teamcity]

This change is 

[rohany]

I'm hesitant to accept this because of my concern on the issue. I'll review it once we come to a conclusion there

[ajwerner]

And 12 months later, I think we're up for this. Let's get @vy-ton to sign off. Any reason not to? The accepted workaround in postgres is to drop in plpgsql which isn't something we can do at this point. @ekalinin are you up for rebasing this?

[vy-ton]

+1 to adding this, thanks for the contribution!

[ekalinin]

And 12 months later, I think we're up for this. Let's get @vy-ton to sign off. Any reason not to? The accepted workaround in postgres is to drop in plpgsql which isn't something we can do at this point. @ekalinin are you up for rebasing this?

Sure, will do it!

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[ekalinin]

There's a better PR (#71257) for the original issue.
I think it would be better to continue work in there.