Add new parameter to GCP recommendation

[anbrenna]

The API that returns GPC recommendation related to disks has to be enhanced with a new parameters to indicate how will be easy implement the recommendation.
The average percentage of the values reported in the recommendation should be calculated.
4 range should be defined

• 0-25
• 25-50
• 50-75
• 75-100

[afumagalli98]

the resolution level column is not very clear to understand, and must be renamed in "optimization score" and the value is not a string but an integer.
this field is the average of the details values and must be calculated as follows:

• optimization score = 100 - avg.percentage

change the field name and behavior also in the export API