Add validation for autoscale params in pagopa-dx/azure_api_management

[BurnedMarshal]

Hi, I want to suggest an enhancement for the module pagopa-dx/azure_api_management.

When APIM has zone redundancy enabled, the autoscaler fails if the number of instances is not a multiple of two. This error occurs in production and prevents the APIM from scaling. It would be useful to add strict validation on the autoscale variable inputs of the module to avoid erroneous configurations by teams.
The new validation should be enforced for all the following parameters:

• minimum instances
• maximum instances
• default instances
• scale-out instance count
• scale-in instance count

Thanks.