sql: Refactoring structured.proto. Renaming ColumnType Kind enum to SemanticType.

[m-schneider]

The current use of the the word kind to denote type, in the ColunmType message
is confusing. The upcoming changes to intoduce new enums to seperate semantic
types from visible types will only make this worse.

Closes #10705

[cockroach-teamcity]

This change is 

[knz]

Oh my. A dream come true. @arjunravinarayan come check this out!

Masha:

• please update the existing RFCs that mention the term "kind" to be consistent with the new naming ( can see two at least)
• please also rename the field parser.DOid.kind
• extend your commit message (and possibly the PR description) with a rationale for this change. You can take inspiration from our discussion on the other PR.

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[rjnn]

Wonderful! Will give a thorough review shortly, but a quick note that "Closes #10705" would be useful in the commit message.

[rjnn]

@knz, what is the "other PR"?

[m-schneider]

Raphael:

• Updated every RFC that references kind.
• renamed Doid.kind.
• Extended the commit message with some inspiration from sql: Adding support for integral aliases. #16548 and sql: bikeshed on renaming ColumnType, ColumnType_Kind, ColumnType_Kind_value #10705.

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Comments from Reviewable

[m-schneider]

Arjun, added the issue to the commit message.

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Comments from Reviewable

[knz]

@arjunravinarayan the renaming discussion started in #16548 (comment)

[knz]

@m-schneider you need to write literally "Fixes #10705." or "Closes #10705." on a line by itself in either the PR description or the commit message for Github to pick it up when you'll merge this.

[knz]

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Comments from Reviewable

[rjnn]

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Reviewed 28 of 29 files at r1, 6 of 6 files at r2.
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[m-schneider]

@knz, didn't know that thanks for the tip!
Done.

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