Auto-register Joe Bot on the Platform

Goal

To simplify Joe Bot installation, provide automatic registration on the Platform for the WebUI communication channel.

TODO / How to implement

automatically register Bot on the Platform if configuration provided:
- extend Joe configuration - 2
```
  app:
    host: "127.0.0.1"
    port: 2400
    minNotifyDuration: 60s
    debug: true
  
  registration:
    enable: true
    publicURL: "https://joe.example.com"

  platform:
    ...
```
- validate configuration(channel mapping configuration, signing secret, project, public URL) - 2
- add methods for Platform client (the Joe Bot repository - /pkg/services/platform/platform.go) - 6
  - registerInstance
    - send a request https://postgres.ai/api/general/rpc/joe_instance_create
    - keep a Joe Bot instance ID given from the Platform (pkg/connection/webui/assistant.go)
  - deregisterInstance - send a request https://postgres.ai/api/general/rpc/joe_instance_delete
- deregister instance on stopping (graceful shutdown) - 4

Acceptance criteria