feat(standardized-auth): add an acme sample for standardized app auth

[WilliamBergamin]

Summary

This pull request adds a standardized-auth sample: Acme, an app that connects user accounts with the Slack-owned Connect and Manage connection controls. Rocket skates, anvils, and giant magnets not included.

• Bolt's installation store is the source of truth: a user is connected when it holds their user token.
• The OAuth success hook reports a new connection with apps.user.connection.requestAuth, which also redraws an open auth modal, and refreshes App Home.
• Disconnect forgets the user token, reports the user as disconnected, then revokes the token; tokens_revoked forgets users who remove the app.
• user_connection_status_requested is answered with apps.user.connection.update.
• App Home renders the auth_provider block.
• Each listener file registers itself with @app.action or @app.event, and app.py imports every file under src/listeners at startup, so a new listener only needs a new file.
• The user_connection listener, for the Connect on the app profile, is registered, but it won't fire until Bolt routes the user_connection payload to .action listeners.
• Pins slack_sdk to python-slack-sdk main by commit until the next release.

Testing

In a workspace where standardized app authentication is available:

1. Run ngrok http 3000 and replace 1234-56-78-90-0.ngrok-free.app in manifest.json.
2. Run slack install --environment local, then slack env init with the client ID, client secret, signing secret, and your ngrok URL as BASE_URL.
3. Run slack run and open https://<ngrok host>/slack/install to install the app.
4. Open the app's Home tab and confirm the "Acme account" row shows Connected as @you.
5. Click Disconnect and confirm the row shows Connect and the user token is revoked.
6. Click Connect, approve in the new tab, and confirm the row shows Connected again.
7. Remove the app's access in your Slack settings and confirm App Home shows Connect.

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