add block_forever example

update example READMEs

add script entry point for block_forever example
add poe task block-forever

examples/block_forever/README.md

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+## About
+
+This example demonstrates the following:
+
+-   How to register callback functions for different kinds of events.
+-   How to view the logs emitted by the streamlabsio library.
+-   How to use the raw data returned by the callback functions.
+-   How to use {Client}.wait() to block the main thread indefinitely.
+
+## Configure
+
+The script expects the Streamlabs token to be loaded into the environment with key `STREAMLABS_TOKEN`.
+
+If you're running the script with `poetry poe block-forever` then poe is configured to load a `.env` file in the root of the repository.
+
+## Use
+
+Run the script and trigger any of the events with `Test Widgets` in the Streamlabs GUI.