Fix busted tab completion of extension versions.

Fix busted tab completion of extension versions.

In 02b8048ba I (tgl) got rid of the need for most tab-completion queries
to return pre-quoted identifiers.  But I over-hastily removed the
quote_ident call from Query_for_list_of_available_extension_versions*
too; those still need it, because what is returned isn't an identifier
at all and will (almost?) always need quoting.

Arguably we should use quote_literal here instead.  But quote_ident
works too and people may be used to that behavior, so stick with it.

In passing, fix inconsistent omission of schema-qualification in
Query_for_list_of_encodings.  That's not a security issue per our
current guidelines, but it ought to be like the rest.

Jeff Janes

Discussion: https://postgr.es/m/CAMkU=1yV+egSYrzWvbDY8VZ6bKEMrKbzxr-HTuiHi+wDgSUMgA@mail.gmail.com