AysyncHTTPClient support for linux

[JonasProgrammer]

As discussed in #127, the current workaround is not really feasible for async applications.

This PR includes an AsyncHTTPClient trait that does the following:

• conditionally depend on AsyncHTTPClient
• conditionally include the AsyncHTTPClient trait for EventSource
• use typealiases to distinguish the session types

When untraited, the library should work as-is. Whith the trait active, the interface is changed to consume an HTTPClient as the session type.
I tried to change the business logic as little as possible, mostly implementing URLSession's extensions for HttpClient.

[KotlinFactory]

Any updates on this perhaps?

[JonasProgrammer]

Btw, as it stands now, the increased deadline fixes some of the timeouts, but not all of them. The existing HttpClient.shared instance, which is the default session with the enabled trait, has a read time out of only 90s and cannot be reconfigured.

Internally, we use modified timeouts along with another shared singleton session, but this is not ideal (AsyncHTTPClient does not expose the means of creating a proper singleton that cannot be shut down as they do with their own .shared singleton), thus not included in the PR.

If you have any suggestions regarding default timeouts etc., I'd be happy to improve the PR in that direction.