… API

Fix incorrect JSON field names on HostedRunner and HostedRunnerImageDetail
structs that don't match the GitHub API response, and rename the Go struct
fields to align with the API spec.

- HostedRunner: ImageDetails/image_details -> Image/image
- HostedRunnerImageDetail: SizeGB/size_gb -> Size/size

BREAKING CHANGE: Renamed Go struct fields (ImageDetails -> Image,
SizeGB -> Size) and their JSON tags. The old JSON tags never matched the
API, so this is effectively a bugfix.

Supersedes google#4102