spanner: Request to skip binary compatibility check for codegen PRs

[skuruppu]

We recently published some additions to the APIs for query optimizer version and backups support. The generated PRs all fail on the binary compatibility check because we add methods to existing interfaces.

Examples are #84 and #97.

In #84, I added clirr-ignored-differences.xml to make the compatibility check happy. I will have to do the same for #97

If there's a way to exclude sub-directories from this check, that would work.