Migrate proxy logic to the API package (#4216)

This allows arbitrary programs to have an easy mechanism to make
connections through Boundary using Go. Internal cluster tests and test
helpers have been migrated to use this package, as well as the `connect`
command.

---------

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

internal/proto/controller/api/resources/targets/v1/target.proto

@@ -371,6 +371,9 @@ message SessionAuthorization {
   // Output only. The marshaled SessionAuthorizationData message containing all information that the proxy needs.
   string authorization_token = 90 [json_name = "authorization_token"]; // @gotags: `class:"secret"`
 
+  // Output only. The connection limit being applied to this session. -1 means unlimited. This is not actually enforced on the client side but it provides for better listener handling by including it.
+  int32 connection_limit = 95 [json_name = "connection_limit"];
+
   // Output only. The endpoint address that the worker will connect to, useful for setting TLS parameters.
   string endpoint = 100; // @gotags: `class:"public"`