use signle gomod

[schoren]

This PR removes the go.work file and centralizes all go deps in a single go.mod at the project root. This means that the root package is now github.com/kubeshop/tracetest. Key benefits:

1. Automatic dep version sync betweeen CLI, server, agent and any future project
2. no need to import different parts throug go mod (for example importing server on CLI)
3. no more go.work diff because of IDEs

All the builds are still working, so this is a very "internal" change that shouldn't affect anything other than go's dep management