Refactor to use single port

[wass3r]

Currently we expose three ports, for:

1. Prometheus (8080)
2. Slack Events API Receiver (3000)
3. Slack Interactions API Receiver (4000)

Consider refactoring to use a single port. Provide an ability to register a handler for any path and by default register handlers for the above, if applicable.

Also, add a configuration option to set the port the user wants to use.

This will reduce number of ports to expose/manage and allow easier deployments in some cases.

[wass3r]

will take the opportunity to also move from gorilla/mux to something else since that is now archived. currently, thinking of go-chi/chi since it's close and offers some other niceties such as no dependencies, etc. however, maybe just sticking to standard library is sufficient.

[Nepo26]

Hey @wass3r, how is the development? Any timeline on this? Need help with anything?

[wass3rw3rk]

@Nepo26 thanks for the interest. not as fast as i hoped :) i had a refactor started that would allow each remote to register endpoints if needed and also set up common ones for metrics/health and then start (one) server with any configured endpoints; i can't commit to a deadline, but i will try to revive that here!