Add proxy configuration to ConnectionPool.

[tomchristie]

Closes #973

• Documentation.
• Implementation.
• Tests.
• Changelog.
• Deprecate HTTPProxy/AsyncHTTPProxy/SOCKSProxy/AsyncSOCKSProxy classes.

This would allow for some refactoring cleanup, eg the following...

https://github.com/encode/httpx/blob/eeb5e3c2a3ff2403ec47b5926715ecd61143d92d/httpx/_transports/default.py#L148-L208

Could become...

proxy_config = httpcore.Proxy(
    url=proxy.url,
    auth=proxy.auth,
    headers=proxy.headers,
    ssl_context=proxy.ssl_context,
) if proxy else None

self._pool = httpcore.ConnectionPool(
    ssl_context=ssl_context,
    proxy=proxy_config,
    max_connections=limits.max_connections,
    max_keepalive_connections=limits.max_keepalive_connections,
    keepalive_expiry=limits.keepalive_expiry,
    http1=http1,
    http2=http2,
    uds=uds,
    local_address=local_address,
    retries=retries,
    socket_options=socket_options,
)

It also paves the way for us pushing a use_system_proxies config into httpcore.ConnectionPool.

This would be beneficial as we can then drop the gnarly approach of "system proxies are handled through mounting different connection pool instances" in httpx.

https://github.com/encode/httpx/blob/eeb5e3c2a3ff2403ec47b5926715ecd61143d92d/httpx/_client.py#L658-L692