Convert to YAML the ecs mappings static file

[mrodm]

Relates #1018

Convert to YAML the file used as a static file to include the ECS mappings. Usually files used in packages are using YAML and moving to this format it will allow us to add comments to some dynamic template if needed for instance.

loadEcsMappings() function was using already yaml library to unmarshal the string:

func loadEcsMappings() (ecsTemplates, error) {
	var ecsMappings ecsTemplates
	err := yaml.Unmarshal([]byte(staticEcsMappings), &ecsMappings)
	if err != nil {
		return ecsMappings, err
	}
	return ecsMappings, nil
}

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Name	Metrics % (covered/total)	Diff
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