Wasm build by adding default-client feature gate

[wooden-worm]

This enables wasm build by adding a new default-client feature and putting all wasm incompatible code behind it.

On wasm targets users can supply their own Service<Request<String>, Response = Response<B>> + Send + 'static.

Fixes #366 #224

[XAMPPRocky]

What makes this one incompatible with WASM?

[wooden-worm]

It depends on OctocrabBuilder::default().build(), which is not available on wasm32. Another approach would be having 2 different OctocrabBuilder::default() impls.

I also found another issue with building for wasm: Buffer::new() here depends on tokio. To make it work on wasm I had to changed to Buffer::pair, pass in the executor and spawn the worker task.

[XAMPPRocky]

Yeah, I think I'd prefer having default client that works on WASM.

[wooden-worm]

I re-checked the code and realized why I didn't go with 2 default impls in the first place: there's currently no good default Svc on wasm. An empty/do-nothing one would be pointless here, since the STATIC_INSTANCE would not work anyway. Bringing in a working Svc for wasm looks like the better option, and reqwest is a great candidate. However it's still using an old version of http seanmonstar/reqwest#2059

I made a half-working implementation using gloo_net, however it involves manually construct gloo_net::Request from http::Request<String>, making the code ugly. Soon after testing a few endpoint, I realize the github rest API would not work for my use case, so had to switch to its graphql API, so I didn't finish the wasm http client tower::Service implementation.

[XAMPPRocky]

Yeah, reqwest is a no go, because it does not support using or getting http::{Request, Response}, reqwest returns its own types and in order to be Sans-IO and client agnostic, octocrab has to support http types, not reqwest types.