backup: add BACKUP/RESTORE SCHEMA syntactic sugar over db.schema.* target

RESTORE TABLE db.schema.* ... is documented/named as a table-level restore, but the * wildcard actually broadens it to mean "restore the whole schema" — tables, views, UDFs, and UDTs. This is a special-cased, undocumented behavior of * rather than a normal pattern glob, and it's confusing: the keyword says TABLE but the semantics are "everything in this schema."

More context around related papercuts: https://cockroachlabs.slack.com/archives/C2C5FKPPB/p1787751068070069?thread_ts=1787342316.494509&cid=C2C5FKPPB

Jira issue: CRDB-67629

Start by reviewing the existing RESTORE TABLE db.schema.* behavior and the related Slack discussion and Jira issue CRDB-67629. Define how BACKUP/RESTORE SCHEMA should map to the existing schema-wide target, including tables, views, UDFs, and UDTs; done means the new syntax has matching documented semantics and coverage for the affected behavior.

Written by the indexing model from the issue text.