The SUSPENDED job status is a new ExecutionGraph state which can be reached from all
non-terminal states when calling suspend on the ExecutionGraph. Unlike the FAILED,
FINISHED and CANCELED state, the SUSPENDED state does not trigger the deletion of the
job from the HA storage. Therefore, this state can be used to handle the loss of
leadership or the shutdown of a JobManager so that the ExecutionGraph is stopped but
can still be recovered. SUSPENDED is also a terminal state but it can be differentiated as
a locally terminal state from FAILED, CANCELED and FINISHED which are globally
terminal states.

Add test case for suspend signal

Add test case for suspending restarting job

Add test case for HA job recovery when losing leadership

Add online documentation for the job status

Add ASF license header to job_status.svg

Not throw exception when calling ExecutionGraph.restart and job is in state SUSPENDED

This closes #2096.