tracing: make otel dependency optional for rego+topdown (#4127)

This follows the same approach as the wasm feature: by default, importers
of

    github.com/open-policy-agent/opa/rego
    github.com/open-policy-agent/opa/topdown

will not get a transitive dependency on the otel libraries.

In terms of functionality, nothing changes for the server and runtime.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>