Split HTTP implementation to its own package

[ricardoboss]

Because dart_seq ships with an implementation of SeqClient, the SeqHttpClient, the whole package has a dependency on http, which it really shouldn't have.

Rather sooner than later, I want to separate the HTTP client implementation into its own package. This would mean the setup is more complicated (users will have to install dart_seq AND dart_seq_http_client) but will allow for better control over (transitive) dependencies.

I ran into a problem related to this one when I published v0.0.3 (6895ef6) just to downgrade the http dependency because of a different project where I use dart_seq. This really shouldn't be necessary.