chore(sidecar): expose a version endpoint

[merklefruit]

As part of our next releases, we are going to start using the Sidecar's CARGO_PKG_VERSION to reliably keep track of versions of the software that node operators are running. Ideally, this should also be available as a header on a per-request basis.

This issue consists in adding this functionality with both a version API endpoint and an HTTP header x-sidecar-version.

A more generic approach could be to introduce a more elaborate Capabilities enum along with the version, similar to Ethereum's engine api specs, but I think this is overkill for our current needs.