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>

api/proxy/testing.go

@@ -0,0 +1,23 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package proxy
+
+import (
+	"context"
+	"crypto/tls"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestClientTlsConfig is designed to allow tests to obtain the TLS
+// configuration that would be used by the proxy in order to make specific calls
+// to workers during tests.
+func TestClientTlsConfig(t *testing.T, authzToken string, opt ...Option) *tls.Config {
+	proxyClient, err := New(context.Background(), authzToken, opt...)
+	require.NoError(t, err)
+	tlsConf, err := proxyClient.clientTlsConfig(opt...)
+	require.NoError(t, err)
+	return tlsConf
+}