Define gRPC interface files for a workspace API

[csweichel]

• define proto files covering services to list workspaces, create and start workspace, and listen to status updates (including pre-builds and image builds)
• gather experiences with/revise API design guidelines from the design doc
• define and separate re-usable concepts, e.g. for paging, structured errors
• generate implementations in TypeScript, Go and Java

Part of #7900