Feature Request: manual ack option

Feature

It would be nice if bolt provides an option to manually send an ack by user.
Currently, bolt automatically sends an ack for the Events API requests.
https://github.com/slackapi/bolt-js/blob/e35ca5ca562213c804cd443d2117c02620e69272/src/App.ts#L1173-L1175

Purpose

I use the socket mode client (@slack/socket-mode) for development in the multiple environments. My client sends an ack only if the message is for the corresponding environment.

For example,

• For the permanent environment, the client sends an ack if it receives a message without any prefix. e.g., @app Hello
• For an ephemeral environment, the client sends an ack if it receives a message with the prefix. e.g., [pr-123] @app Hello

In my experience, we should not use this pattern for permanent environments, because it causes a message delay. Slack retries to send a message until any client sends an ack.

I think this pattern is still useful if we want to test the client in an ephemeral environment.

Start in src/App.ts at lines 1173-1175, where Bolt automatically acknowledges Events API requests, and read the surrounding request-handling flow. Determine how a manual acknowledgement option would affect the socket mode client described in the issue. Done means users can choose manual acknowledgement while the existing automatic behavior remains available.