Add support for Disk Template Version list/get operations

[jimeh]

Add support to the existing DiskTemplatesClient for fetching list of versions,
and details for a specific version.

API documentation:
https://developers.katapult.io/api/docs/latest/groups/disk_template_versions/

I would suggest adding the following methods to DiskTemplatesClient:

GetVersions(ctx context.Context, dt *DiskTemplate) ([]*DiskTemplateVersion, *Response, error)
GetVersion(ctx, id string) (*DiskTemplateVersion, *Response, error)