feat: add the /aitasks/prompts endpoint (#18464)

Add an endpoint to fetch AI task prompts for multiple workspace builds
at the same time. A prompt is the value of the "AI Prompt" workspace
build parameter. On main, the only way our API allows fetching workspace
build parameters is by using the `/workspacebuilds/$build_id/parameters`
endpoint, requiring a separate API call for every build.

The Tasks dashboard fetches Task workspaces in order to show them in a
list, and then needs to fetch the value of the `AI Prompt` parameter for
every task workspace (using its latest build id), requiring an
additional API call for each list item. This endpoint will allow the
dashboard to make just 2 calls to render the list: one to fetch task
workspaces, the other to fetch prompts.

<img width="1512" alt="Screenshot 2025-06-20 at 11 33 11"
src="https://github.com/user-attachments/assets/92899999-e922-44c5-8325-b4b23a0d2bff"
/>

Related to https://github.com/coder/internal/issues/660.

codersdk/client_experimental.go

@@ -0,0 +1,14 @@
+package codersdk
+
+// ExperimentalClient is a client for the experimental API.
+// Its interface is not guaranteed to be stable and may change at any time.
+// @typescript-ignore ExperimentalClient
+type ExperimentalClient struct {
+	*Client
+}
+
+func NewExperimentalClient(client *Client) *ExperimentalClient {
+	return &ExperimentalClient{
+		Client: client,
+	}
+}