add implementation for EtteHttpServer option (flag to expose REST API)

[eperakakis-xm]

I required for ette to work only as a service that populated the PostgreSQL database.

GraphQL can be disabled by EtteGraphQLPlayGround setting. Live subscription can be disabled by using the appropriate EtteMode setting. But, there was no way to disable the REST HTTP server.

This change adds an additional setting, EtteHttpServer in .env file. If it is not true, then the HTTP server won't start. In this case, to avoid the immediate termination of ette, the Run method is blocked.

[itzmeanjan]

Thanks for the PR @eperakakis-xm , I'll take a look.

[itzmeanjan]

I've stopped maintaining ette, so closing this PR. Thanks.