Add HTTPTransport and AsyncHTTPTransport

[lovelydinosaur]

Closes #1302

Two points to note here...

1. I've not added corresponding proxy cases here. We've got two options there...

• Add httpx.ProxyTransport() / httpx.AsyncProxyTransport() classes.
• Add a proxy argument to the existing class. Switch which httpcore instance is used for ._pool depending on that argument. httpx.HTTPTransport(proxy=...), httpx.AsyncHTTPTransport(proxy=...). I think I quite like the way that'd have a lower footprint.

2. I've currently used open-ended kwargs for the advanced options. I'm kinda thinking to switch these out to named uds, local_address, etc... options instead, although kwargs does at least have the benefit of allowing us to add options to httpcore and have them be exposed without any changes in httpx.

[florimondmanca]

Looking good! Is it expecting that keepalive changes are also in here, though?

[lovelydinosaur]

@florimondmanca Okay, so...

I've gone for explicit arguments throughout. (Maxim of: Of two otherwise equally valid design options, choose the more constrained) Note that it highlights the fact that uds, local_address, and retries aren't currently supported by the proxy cases. Perhaps some of them should or not, unsure, but we should discuss that separately either way - we're not backtracking on anything here, just making it more obviously apparent.

I've also added the proxy=... argument directly here. It ends up rather neat, and is the most minimal increase the API surface area, so I reckon we ought to roll with that.

[florimondmanca]

Okay, this is looking neat. 👍

[remingtonc]

This is an exciting addition - any idea when it will be released?