Add connection management for H/2

[srikrsna-buf]

Add connection management for H/2. This adds an abstraction and a default implementation for managing (reusing) HTTP/2 connections across requests.

The keep alive logic is similar to the ones found in other connect implementations and exposes similar options to tweak the behavior.

Unfortunately this also introduces a minor breaking change for users using H/2 with TLS, they will have to pass the security context in a slightly different way:

- createHttpClient(context: context);
+ createHttpClient(
+   transport: Http2ClientTransport(
+     context: context,
+   ),
+ );

Not super happy with the name Http2ClientTransport as we use Transport for protocols.