builtins: implement ST_ForceCollection

[jseldess]

Jesse Seldess (jseldess) commented:

PR: cockroachdb/cockroach#53643

From release notes:

Implemented the geometry [builtin](../v20.2/functions-and-operators.html#built-in-functions] ST_ForceCollection. [#53643][#53643) {% comment %}doc{% endcomment %}

Jira Issue: DOC-758

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