feat: add mocked http request tests

[drbh]

This PR implements two http request tests, and adds helper functions to test JSON requests and expected outputs.

test server::tests::test_echo_inputs_when_skip_generation ... ok
test server::tests::test_return_json_error_on_empty_inputs ... ok

Notes:

• this pr adds tower = "0.4.13" so we can use the tower::util::ServiceExt trait to allow oneshot requests in the tests without starting an HTTP server.
• skip_generation has been added to GenerateParameters as a way to avoid making requests to the router.

@Narsil and @OlivierDehaene please let me know what you think of this approach and if there is a better way to implement tests (the longer term goal is to improve TGI's API and these tests may be helpful when extending the API)

[Narsil]

Could you try not creating any surface just for tests ?

Tests are after all just for tests, they shouldn't ever require the code to change to accomodate for them.

Also adding new dependency is really a big thing imo, we should really refrain from doing so (especially since axum should provide everything we need already).