sql: "default" DCollatedString should be DString objects

[otan]

In Postgres, COLLATE strings are just a typmod on the string type.
In CRDB, COLLATE is an entirely new type, with its own family and handling logic.

Unfortunately, this presents a problem when using "default" collation, which should store no typmod for the string object for collate. We however have pidgeonholed all our logic such that x COLLATE y forms a CollatedString type.

This is problematic for DEFAULT collations when a user specifies it, as it tries to evaluate the language tag. Since we introduced DEFAULT column definitions in #56598, this is only a problem when someone does x COLLATE "default" as a value.

Jira issue: CRDB-2845

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[rafiss]

Does this cover #50734 or should we keep that open separately?

[otan]

kind of separate, this one covers a more general code change whereas sql: support COLLATE "default" in SELECT queries covers a specific use case working.