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.