sql/parser: Added current_schema keyword support.

[CMajeri]

current_schema can be used as an alias for the current_schema() builtin.
It is used by some applications like Keycloak.

[cockroach-teamcity]

This change is 

[CLAassistant]

All committers have signed the CLA.

[jordanlewis]

Thanks for the contribution! This looks correct to me, but it's missing tests. Please add a test to sql/parser/parse_test.go - there should already be a similar one for current_timestamp, for instance.

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[jordanlewis]

Great - now could you please squash the two commits and rebase them instead of including merge commits? Then I think we'll be good to go.

[CMajeri]

Hi, I already squashed them didn't I? I can only see one commit in my PR.

[jordanlewis]

Yes, it looks good now. Before it showed several commits, perhaps it was a GitHub ui blip.

[knz]

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Reviewed 4 of 4 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[knz]

@CMajeri thank you for your contribution!! -- Sent from my Android device with K-9 Mail. Please excuse my brevity.