chore: refactor dynamic parameters into dedicated package (#18420)

This PR extracts dynamic parameter rendering logic from
coderd/parameters.go into a new coderd/dynamicparameters package. Partly
for organization and maintainability, but primarily to be reused in
`wsbuilder` to be leveraged as validation.

coderd/coderdtest/stream.go

@@ -0,0 +1,25 @@
+package coderdtest
+
+import "github.com/coder/coder/v2/codersdk/wsjson"
+
+// SynchronousStream returns a function that assumes the stream is synchronous.
+// Meaning each request sent assumes exactly one response will be received.
+// The function will block until the response is received or an error occurs.
+//
+// This should not be used in production code, as it does not handle edge cases.
+// The second function `pop` can be used to retrieve the next response from the
+// stream without sending a new request. This is useful for dynamic parameters
+func SynchronousStream[R any, W any](stream *wsjson.Stream[R, W]) (do func(W) (R, error), pop func() R) {
+	rec := stream.Chan()
+
+	return func(req W) (R, error) {
+			err := stream.Send(req)
+			if err != nil {
+				return *new(R), err
+			}
+
+			return <-rec, nil
+		}, func() R {
+			return <-rec
+		}
+}