Create necessary factories only when using HTTP transport

[ghost]

This PR does the changes discussed in #741.

When setting a custom transport, the client builder will not create any factories used by the HTTP transport anymore. These prevented using a custom transport which did not rely on HTTPlug (because these relied on class discovery).

Closes #741.

[Jean85]

I do like a lot this change! Good job!

I would like to have a regression test for this, but I fear that's not possible.. @ste93cry WDYT?

[ste93cry]

I would like to have a regression test for this

Some tests of the client builder class already checks whether the HTTP transport that is created uses the custom factories, so no need for a regression test. PR is fine for such change, the only thing I would like to explore (it's not blocking btw) is whether it makes sense to expose the set*Factory methods in the client builder. Since the client is agnostic of the transport, the builder should not know anything regarding a specific transport type which in this case is HTTP

[Jean85]

I agree with the possibility of removing the factory setters. But we can address that in a separate PR.

Thanks @CharlotteDunois for this one, merging!