Refactor and separate nostr part of the code

Currently all the echo handlers lie in the nostr file and even the service is defined there. It would be great if we can move the service and it's handler functions elsewhere (ideally to `service.go`) and only keep nostr related parts of the code in `internal/nostr/nostr.go`

Also, kinda related: https://github.com/getAlby/http-nostr/pull/72#discussion_r1620228014