Prompts.get() can now fetch by label (e.g. "production")

Minor changes

• 1653bcb Add a label option to Prompts.get() to fetch the prompt version a label (e.g. production) currently points to. Labeled fetches are cached separately, and PromptResult carries the resolved label. Requires a PostHog version with prompt labels; older servers ignore the parameter and return the latest version. — Thanks @jurajmajerik!