sql: Consider supporting a version of rename table that lets you replace a different table

[a-robinson]

The idea is that syntax along the lines of ALTER TABLE foo REPLACE WITH bar would allow you to atomically drop foo and rename bar to foo. This came up in discussion on #12158 (direct comment link), in which we want to replace system.eventlog with a table that only differs in one of the primary key columns.

Alternatively, fixing #12123 such that a user could DROP a table and RENAME over it within a transaction would basically make this pointless.

I don't think we should do this unless other situations come up in which the same idea arises, but I want to make sure we at least track such situations so we can notice the pattern if it arises.

[thoszhang]

Closing in favor of #54562, which we intend to fix for 21.1.