Properly support setting DD_TRACE_TRANSPORT to "none"

[kristjanvalur]

Description

Omit the 'curl' library if DD_TRACE_TRANSPORT is set to "none"

Motivation

There is clearly the intent to be able to not include a default transport, by setting DD_TRACE_TRANSPORT=none.
However, two things were missing:

1. To ignore the bundled curl library from the build
2. to replace the default_http_client_curl.cpp with default_http_client_null.cpp

Integrations that provide their own transport may not want to build and link a whole unused library, along with any runtime library dependencies it makes.

Additional Notes

[dmehala]

Fantastic @kristjanvalur.
This is indeed the intended behaviour. I appreciate your contributions.

[dmehala]

LGTM!
If I recall correctly, when no HTTP client is provided, the library defaults to default_http_client. This could potentially lead to a crash since the library assumes default_http_client is always a valid instance. While it functions, some refactoring would be beneficial to help users avoid unintended issues.

I think the tradeoff is ok, as long as it help you progress on your plugin :)

Thank you!

[kristjanvalur]

I could add a check in the config builder for a nullptr, but since this is an integration issue and not a configuration issue, in my view assuming a non-null ptr (and thus a correctly written integration) is ok.