agent: Use an in-process listener with cache (#12762)

Uses a bufconn listener between consul-template and vault-agent when
caching is enabled and either templates or a listener is defined. This
means no listeners need to be defined in vault-agent for just
templating. Always routes consul-template through the vault-agent
cache (instead of only when persistent cache is enabled).

Uses a local transportDialer interface in config.Cache{}. 

Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>
Co-authored-by: Ben Ash <32777270+benashz@users.noreply.github.com>

internalshared/listenerutil/bufconn.go

@@ -0,0 +1,38 @@
+package listenerutil
+
+import (
+	"context"
+	"net"
+
+	"google.golang.org/grpc/test/bufconn"
+)
+
+const BufConnType = "bufconn"
+
+// BufConnWrapper implements consul-template's TransportDialer using a
+// bufconn listener, to provide a way to Dial the in-memory listener
+type BufConnWrapper struct {
+	listener *bufconn.Listener
+}
+
+// NewBufConnWrapper returns a new BufConnWrapper using an
+// existing bufconn.Listener
+func NewBufConnWrapper(bcl *bufconn.Listener) *BufConnWrapper {
+	return &BufConnWrapper{
+		listener: bcl,
+	}
+}
+
+// Dial connects to the listening end of the bufconn (satisfies
+// consul-template's TransportDialer interface). This is essentially the client
+// side of the bufconn connection.
+func (bcl *BufConnWrapper) Dial(_, _ string) (net.Conn, error) {
+	return bcl.listener.Dial()
+}
+
+// DialContext connects to the listening end of the bufconn (satisfies
+// consul-template's TransportDialer interface). This is essentially the client
+// side of the bufconn connection.
+func (bcl *BufConnWrapper) DialContext(ctx context.Context, _, _ string) (net.Conn, error) {
+	return bcl.listener.DialContext(ctx)
+}