sql: allow schema changes on tables in state: TableDescriptor_ADD

[vivekmenezes]

A new table descriptor is placed in the non-public TableDescriptor_ADD state
when it refers to another table through an FK or interleaved reference,
and can only be made public once the other tables back reference is
visible across the cluster.

This change allows schema change operations on such non-public tables,
while preserving the non-public behavior for table leases.

fixes #13505

[cockroach-teamcity]

This change is 

[dt]

though I'm a little uneasy about the common mustGet.* helpers being willing to return non-public descriptors by default -- while the current check in data_source might cover current uses, another might sneak in if someone isn't expecting these semantics / doesn't carefully read the comment. How much uglier would it be for them to return desc, filterTableState(desc) and then let schema change callsites decide if they want proceed on errTableAdding ?

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[vivekmenezes]

I thought about what would be safest change to make here and felt that passing in a boolean allowAdding makes it explicit, so that's what I've done. Turns out dan just merged a change for show_fingerprints that should disallow the use of a table in the ADD state and that could have easily slipped through the cracks without the boolean flag. Thanks for your feedback.

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[vivekmenezes]

Let me add a few more tests to this PR before merging it.

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Review status: 0 of 9 files reviewed at latest revision, all discussions resolved, some commit checks pending.

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[vivekmenezes]

I've added a few more tests to this PR. Thanks

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Review status: 0 of 9 files reviewed at latest revision, all discussions resolved, some commit checks pending.

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[dt]

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Review status: 0 of 9 files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable