buildctl: Provide --wait-for-ready option

Added --wait-for-ready timeout to buildctl's global options. See below for behavior.

Implemented a WithWaitForReady client option method that blocks remote calls should the connection ever enter a transient failure state and waits up to the given duration until a connection is ready again before retrying the call. Note that if data has been written to the connection, retries are not attempted. See grpc.WaitForReady documentation for details.

Also note this option only applies to unary calls other than Solve as the timeout is applied to the context of the call and the latter is too likely to exceed a given timeout.