Create Getters on service's maps

[NicolasMahe]

I saw in quite a lot of files this boring piece of code to access value of maps and return an error if not found: https://github.com/mesg-foundation/core/blob/bd84ad1d54eadfb8aa2bd79ac0c7c68f775f9c03/execution/complete.go#L13-L19

I suggest to create getters that will return the expected data and the error if not found. We should have getters for:

• [ ] service's event. func (service *Service) GetEvent(eventKey string) (*Event, err)
• [ ] service's task. func (service *Service) GetTask(taskKey string) (*Task, err)
• [ ] output of service's task. func (service *Service) GetTaskOutput(taskKey string, outputKey string) (*Output, err)
• [ ] maybe on all map of Service?

@mesg-foundation/core what do you think?

[ilgooz]

This validations also shouldn't be made in Complete(). We should first check data for execution and only then call Complete() with valid data in the api handlers. Passing an invalid data to Complete() doesn't make much sense.

event and execution packages needs to be more agnostic and only do their own jobs.