Implement `schemas.delete` RPC method

[seancolsen]

Fixes #3609

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• [x] I added tests for the changes I made (if applicable).
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Developer Certificate of Origin

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[seancolsen]

@mathemancer heads up that test_get_valid_target_type_strings (an SQL test you wrote in d44c0b8a760886262e85b0e5e6f5e72aa7399589 in #3556) was being flaky for me, including in CI. Sometimes it was passing. Sometimes it was failing with this error:

# Subtest: public.test_get_valid_target_type_strings()
    not ok 1
    # Failed test 1
    #         have: ["text", "numeric"]
    #         want: ["numeric", "text"]
    not ok 2
    # Failed test 2
    #         have: ["text", "numeric"]
    #         want: ["numeric", "text"]
    ok 3
    1..3
    # Looks like you failed 2 tests of 3
not ok 94 - public.test_get_valid_target_type_strings

I fixed the test in 529ffef1c1ffbfcc2e3fd9036cb53a479d6ac47e by making the assertions order-independent. That commit is unrelated to this PR, but I tacked it on here for simplicity's sake. I'm happy to cherry pick this into a separate PR if you prefer. I wouldn't be surprised if we see this flakiness crop up in other PRs until we merge this fix.

[seancolsen]

Ready for re-review @mathemancer