x/build/cmd/gomote: implement the gomote api

[cagedmantis]

This issue tracks the implementation of the gomote API:

• Finalize the API design
• Implement Authenticate endpoint.
• Implement CreateInstance endpoint.
• Implement DestroyInstance endpoint.
• Implement ExecuteCommand endpoint.
• Implement InstanceAlive endpoint.
• Implement ListDirectory endpoint.
• Implement ListInstances endpoint.
• Implement ReadTGZ endpoint.
• Implement RemoveFiles endpoint.
• Implement SignSSHKey endpoint.
• Implement UploadFile endpoint.
• Implement WriteTGZFromURL endpoint.
• Implement WriteFileFromURL endpoint.
• Add the option to disable the IAP audience check.

This is a component of the project to revamp the security model used by gomote #47521
@golang/release

[gopherbot]

Change https://golang.org/cl/356589 mentions this issue: internal/gomote/protos: add a skeleton for a gomote api

[gopherbot]

Change https://golang.org/cl/361098 mentions this issue: internal/gomote, cmd/coordinator: add GRPC gomote server

[gopherbot]

Change https://golang.org/cl/368674 mentions this issue: cmd/coordinator, internal/coordinator/pool: move pool buildlet func

[gopherbot]

Change https://golang.org/cl/368675 mentions this issue: cmd/coordinator, internal/coordinator/log: create coordinator log pkg

[gopherbot]

Change https://golang.org/cl/368676 mentions this issue: cmd/coordinator, internal/coordinator/schedule: create schedule pkg

[gopherbot]

Change https://golang.org/cl/370662 mentions this issue: internal/gomote, internal/gomote/protos: add create instance endpoint