Add support for KEFS / File Storage Volumes

[jimeh]

Blocked by:

• [x] File Storage Volume support in Katapult's Core API. — Status: Done.
• [x] krystal/go-katapult#136

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The ability to manage KEFS / File Storage Volumes in Terraform would be very useful.

Tasks

• [x] Add katapult_file_storage_volume resource, with attributes:
  • id (computed)
  • name
  • data_center (computed)
  • associations (currently, list of VM IDs only)
  • size (in bytes, computed)
  • nfs_location (computed)
• [x] Add katapult_file_storage_volume data source, with attributes:
  • id
  • name
  • data_center
  • associations
  • size (in bytes)
  • nfs_location
• [x] Add katapult_file_storage_volumes data source, returning the list of volumes in the organization.