add endpoint to socketmode Clinet and OptionAPIURL function

[ToshihitoKon]

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Run make pr-prep from the root of the repository to run formatting, linting and tests.

Should this be an issue instead

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Change points

• Added an endpoint member to SocketMode Client
• Added an socketmode.OptionAPIURL function

Motivation

I'm creating an application that acts as a server for a Slack App.
To do that, we need to point the Endpoint accessed from slack-go/slack away from https://api.slack.com.

The slack client provided a way to change the Endpoint URL, while the socketmode client did not. I also found the URL hardcoded in the openAndDial function.
This PR prepares the same OptionAPIURL for socketmode client as for slack client.

I would like opinions on whether this change should go into slack-go/slack.

[ToshihitoKon]

This change was unnecessary.

I figured out that the websocket URL is taken from the result of the apps.connections.open API request and the endpoint of the websocket module is irrelevant.