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

[MarcoPolo]

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.

[MarcoPolo]

@marten-seemann expressed interest in working on this.