add endpoint to socketmode Clinet and OptionAPIURL function

[ToshihitoKon]

Pull Request Guidelines

These are recommendations for pull requests. They are strictly guidelines to help manage expectations.

PR preparation

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

Should this be an issue instead

• [ ] is it a convenience method? (no new functionality, streamlines some use case)
• [ ] exposes a previously private type, const, method, etc.
• [ ] is it application specific (caching, retry logic, rate limiting, etc)
• [ ] is it performance related.

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

• no tests, if you're adding to the API include at least a single test of the happy case.
• If you can accomplish your goal without changing the API, then do so.
• dependency changes. updates are okay. adding/removing need justification.

Examples of API changes that do not meet guidelines:

• in library cache for users. caches are use case specific.
• Convenience methods for Sending Messages, update, post, ephemeral, etc. consider opening an issue instead.

---

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.