Implement runtime-provided periodic timer input for source operators

[phil-opp]

The periodic timer inputs allows operators to periodically produce values. To use the new input types, operators specify an input that maps to dora/timer/secs/X where X is the number of seconds between ticks. In addition to secs, the runtime also support millis for millisecond intervals.

The new dora timer nodes are visualized in the following way:

flowchart TB
  node_1[\node_1/]
  logger[/logger\]
subgraph python-operator [node]
  python-operator/op[op]
end
subgraph ___dora___ [dora]
  subgraph ___timer_timer___ [timer]
    dora/timer/secs/5[\secs/5/]
    dora/timer/millis/100[\millis/100/]
  end
end 
  python-operator/op -- counter as test --> logger
  dora/timer/secs/5 -- tick --> logger
  dora/timer/millis/100 -- dora_time --> python-operator/op
  node_1 -- data --> python-operator/op

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TODO:

• Implement parsing of new dora/timer/... inputs
• Visualize the new timer inputs
• Spawn a thread to send the timer tick messages