Modify http and grpc proxy to receive a listener rather than creating it

[roobre]

Description

This PR changes HTTP an GRPC proxy constructors to receive a listener as an argument, instead of receiving an address and creating it themselves as they used to.

Unblocks: #314

Doing this allows to move most of the startup logic to the constructor. Doing so, removes the potential race condition between Start and Stop.
Fixes: #319

Fixes # (issue)

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make test) and all tests pass.
• I have run relevant e2e test locally (make e2e-xxx for agent, disruptors, kubernetes or cluster related changes)
• Any dependent changes have been merged and published in downstream modules
  

[roobre]

As an interesting side-effect, the following e2e test is returning a different error now:

--- FAIL: Test_Agent (116.51s)
    --- FAIL: Test_Agent/Prevent_execution_of_multiple_commands (21.85s)
        agent_e2e_test.go:328: unexpected error: setting up listener at ":8080": listen tcp :8080: bind: address already in use
            :
FAIL
FAIL	github.com/grafana/xk6-disruptor/e2e/agent	143.144s
FAIL
make: *** [Makefile:36: e2e-agent] Error 1

I believe the reason for this is simply that before, ApplyDisruption was called before the proxy started the listener.

[roobre]

After changes in 1a46933, E2E are passing fine, the error regression is no longer there. Ready to review.

[roobre]

Commits squashed, I'll merge after a final pass of e2e tests passes.