feat: unstable HttpClient Config

[Fishrock123]

This adds an unstable-config feature, with a new Config struct, which can be used to configure any HttpClient which implements support for it.

Currently it supports two features - the most important and most generally supported:

• timeout (Duration)
• tcp_no_delay (bool)

Implementations are provided for async-h1, isahc, and hyper (partial, no no_delay support due to the tls connector).

No serious attempt has been made to add this to the wasm client at this point, since I don't understand well how to even build the wasm client or if it even works anymore with the state of rust wasm web build tools.

This represents the development of the older draft PR: #84
Should address #61

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Possible next targets:

• http_keep_alive on/off (Requires a lot of code shuffling in the h1 client)
• socket ttl (Does not appear to be accessible from hyper)
• max_connections - a slightly haphazard api by the h1 client
  • Isahc has two options for this: https://docs.rs/isahc/1.3.1/isahc/struct.HttpClientBuilder.html#method.max_connections
  • Currently ours is per-host and probably ought to be renamed, and a client-global one added (somehow)
  • Hyper ... doesn't have comparable options? Idk: https://docs.rs/hyper/0.14.7/hyper/client/struct.Builder.html

[Fishrock123]

Updated with an http_keep_alive option.

[jbr]

I think I've mentioned this before, but I'd be curious how tls configuration fits into this — would that be a later enhancement?

[Fishrock123]

Probably - as it stands a third of our configurations are completely unable to use TLS configuration of any kind, because they use dependencies that offer no options, namely:

• hyper-tls
• async-tls (the rustls adaptor, but rustls itself is very configurable)

We could support PEM or DER encoded X.509 certificates for isahc and async-native-tls, and maybe a couple extra options:

• isahc: can set ca certs to trust, ssl ciphers, client cert, or be told to trust invalid things
• async-native-tls: can add ca certs to trust, can be told to trust invalid things, ssl ciphers controlled by tls version flags

EDIT: Turns out I was very wrong and these things do indeed have configuration for this but it's a little hidden.

[Fishrock123]

Ok updated with tls options for the async-h1 client. Different configuration is accepted depending if native-tls or rustls is selected.

This is not implemented for the isahc or hyper clients as those can be set directly and then wrapped, and the complexity of doing that integration fully would be too substantial for this PR.

[Fishrock123]

I'd rather avoid max_connections for this unstable pass, it should really be re-thought for http-client 7.0...

[Fishrock123]

merging this as an unstable feature and releasing it.

[Fishrock123]

Released into Surf: https://github.com/http-rs/surf/releases/tag/v2.3.0