Potential ABI breakage in upcoming minor releases

Hello,

Commit 51ff46de29f67d73549b2858f57e77ada8513369 (backported all the way
back to v12) added a new member to `ResultRelInfo` struct. This can
potentially cause ABI breakage for the extensions that allocate the struct
and pass it down to the PG code. The previously built extensions may
allocate a shorter struct, while the new PG code would expect a larger
struct, thus overwriting some memory unintentionally.

A better approach may have been what Tom did in
8cd190e13a22dab12e86f7f1b59de6b9b128c784, but I understand it might be too
late to change this since the releases are already tagged. Nevertheless, I
thought of bringing it up if others have different views.

Thanks,
Pavan