Add renovate

It would be great to use a tool like renovate to automatically notify and update dependencies which we're using.

Ideally it would track:

• rust toolchain version (set in various Dockerfiles, CI workflows, etc.)
• rust dependencies (in Cargo.tom)
• cargo-pgx version (set in various Dockerfiles, CI workflows, etc.)

We have some examples of how to configure renovate with a go project in the promscale repo.