Add proxy configuration to httpcore.ConnectionPool #973

Proxy configuration is currently supported through two connection pool subclasses...

• SyncHTTPProxy(...)
• SyncSOCKSProxy(...)

We could instead have proxy configuration on the main class...

pool = httpcore.SyncConnectionPool(
    ssl_context=ssl_context,
    proxy=httpcore.Proxy(url, auth, headers, ssl_context),
    ...
)

With the switching handled in create_connection.

Benefits...

• Neater API, with fewer types exposed.
• Opens a pathway to handling system proxies within httpcore rather than at the layer above.

^{* Let's also have a httpcore.Proxy() class here to encapsulate the configuration.}