Update PlatformService adding methods to retrieve individual regions (#231)

- Add `GetGlobalCloudProviderRegion` and `GetCloudProviderRegion` methods
- Extend the `CloudProviderRegion` message with new fields:
  - `namespace`: When the region represents a hybrid cloud environment,
    it's useful to know in what is the k8s namespace where the components
    are deployed.
  - `capabilities`: To indicate if certain capabilities are available in
    the region. Currently it only includes information about volumes,
    but in the future we could add more like gpu, private link, etc.

Author: Toni Reina