x/build/cmd/gomote: add gomote session manager package

[cagedmantis]

Add a component which manages gomote instances. This component should:

• Manage the lifecycle of gomote sessions (handle timeouts)

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

[gopherbot]

Change https://golang.org/cl/361097 mentions this issue: internal/coordinator/remote: refactor remote buildlet session

[gopherbot]

Change https://golang.org/cl/361096 mentions this issue: internal/coordinator/remote: copy existing remote buildlet management

[cagedmantis]

This issue has been edited. We no longer plan to manage SSH keys in the coordinator. We've shifted over to using certificates for authentication. This work has been completed.