Add Clock block

[User344]

A simple block with single output that periodically sends current timestamp to output port.
Support CLI, random duration and fraction of seconds:

protoflow execute Clock fixed=2
protoflow execute Clock fixed=0.5
protoflow execute Clock random=1..5
protoflow execute Clock random=0.5..1.5

I am honestly not sure how to properly do mermaid diagram so I would greatly appreciate if someone made sure it is correct or helped me fixed it if it's not.