[Feature] Manage synthetics private locations in a space-aware manner

[frconil]

Is your feature request related to a problem? Please describe.

Initially, private locations were not space aware, which caused issues in the terraform providers. This was addressed in #733

Recent work in elastic/kibana#195874 and planned work in elastic/kibana#199976 aim to make this possible, and as such it would be helpful to be able to manage the (new) feature with terraform

Describe the resource you would like to have implemented.

The API in question is the Create Private Location API

cc @shahzad31 @biscout42

[biscout42]

Thanks, @frconil .

@shahzad31 , FYI: we would need to think about migration path for our customers. When we will introduce space-aware private locations, how customers with already existing terraform state (current ID of private location) will be able to migrate to the new functionality?