multidim interop: Allow impl repos to test against only the latest version

As a way to speed up CI tests and mitigate a bit of #229 , one idea would be to support only building and loading the latest version of other implementations when run from an implementation's repo. For example, js-libp2p would only build and test against the latest JS, Go, and Rust.

This should help use less space, and could speed things up a bit.

In the test-plans repo we would still test against older versions. This means that when a new implementation version is released, it'll still benefit from testing against older versions, just not with the same per-PR fidelity. I think that's an okay tradeoff to make since the alternative of not even building because we are past the GH actions limit isn't viable.

Start by tracing how implementation repositories select versions to build and load, then compare that flow with the test-plans repository's matrix. The change is complete when implementation repositories test only the latest versions of other implementations while test-plans continues covering older versions.

Written by the indexing model from the issue text.