Create abstraction for Sensu mutators

[fguimond]

Create an abstraction to execute Sensu Mutators (https://docs.sensu.io/sensu-go/5.5/reference/mutators/). Mutators accept a Sensu event, transform the event and returns the output JSON.

The first version of this repository contains an abstraction for the common Sensu handles logic. The abstraction is responsible to

• Read and validate the Sensu event from stdin
• Read the arguments from the command line, environment variable or Sensu event
• Execute the user provided validation logic
• Execute the user provided handler logic

We want to implement something similar for mutators.

The similarities and differences are:

• Mutators and handlers both read a Sensu event from stdin
• Mutators and handlers both read command line and env variable arguments
• Do we want to support configuration overrides in the mutators? @calebhailey
• Mutators return the transformed event's JSON to stdout or stderr
• Mutators return a 0 exit status code upon success or any other in case of failure

How to implement

1. See the way options are defined at https://github.com/sensu-skunkworks/sensu-aws-ec2-deregistration-handler/blob/sensu-auth/main.go#L31. Very similar approach but no need to have the Path property since it defines where to look in the Sensu Event. Dependent on the support of configuration overrides
2. Create a new sensu.NewGoMutator method instead of sensu.NewGoHandler https://github.com/sensu-skunkworks/sensu-aws-ec2-deregistration-handler/blob/sensu-auth/main.go#L31
3. The validation method signature should be the same - https://github.com/sensu-skunkworks/sensu-aws-ec2-deregistration-handler/blob/sensu-auth/main.go#L31
4. The execution method should return an interface object with JSON annotations, or an error - https://github.com/sensu-skunkworks/sensu-aws-ec2-deregistration-handler/blob/sensu-auth/main.go#L31 Signature might be something like func(event *types.Event) (*types.Event, error)
5. If no error is present the exit status code is automatically set to 0, otherwise it is set to 1

Unit tests are required.

[fguimond]

Sensu Go Plugin Library

[fguimond]

This has been implemented in https://github.com/sensu/sensu-plugins-go-library/pull/5.