Add execution worker pool

[arbulu89]

Executions worker pool. It follows the same approach we used in the Collector data pipeline in the past.

The PR adds:

• New /api/execute endpoint to schedule new execution events. By now, it only accepts a {"id": 1} kind of struct. We will decide the contract between the server and runner later on, but I assume an ID will come.
• Once execution requests are coming, they are schedule. I have updated the channel to be a buffered one. This way, we can control if the channel queue is empty or not, and we can return back not possible to execute kind of answers. The channel size is not the workers number. It is only used to know if we need to reply with errored message to the POST request or not
• Some additional test related code, as setup the test environment.

The PR doesn't implement yet the checks execution. It just have a dummy Execute function.

[fabriziosestito]