http: making http upstreams pluggable (#11327)

Mainly moving code, but finishes up the series of pluggable upstream PRs.

Additional Description: This unhides the configurable extension point to the cluster for selecting a connection pool and creating an upstream, which can be used for custom business logic in upstream creation.

Risk Level: medium (router refactor)
Testing: with prior PRs
Docs Changes: inline with APIs
Release Notes: added

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ 8a9d6155601846d2d256defd09a3c408c6eb4386

envoy/config/cluster/v3/cluster.proto

@@ -818,7 +818,6 @@ message Cluster {
   // of 100 would indicate that the request took the entirety of the timeout given to it.
   bool track_timeout_budgets = 47;
 
-  // [#not-implemented-hide:]
   // Optional customization and configuration of upstream connection pool, and upstream type.
   //
   // Currently this field only applies for HTTP traffic but is designed for eventual use for custom

envoy/config/cluster/v4alpha/cluster.proto

@@ -819,7 +819,6 @@ message Cluster {
   // of 100 would indicate that the request took the entirety of the timeout given to it.
   bool track_timeout_budgets = 47;
 
-  // [#not-implemented-hide:]
   // Optional customization and configuration of upstream connection pool, and upstream type.
   //
   // Currently this field only applies for HTTP traffic but is designed for eventual use for custom