WIP: add initial signals class definitions

[harshanarayana]

Implementation for #1630

[codecov]

Codecov Report

Merging #1770 into master will decrease coverage by 0.81%.
The diff coverage is 68.67%.

@@            Coverage Diff             @@
##           master    #1770      +/-   ##
==========================================
- Coverage   91.95%   91.14%   -0.82%     
==========================================
  Files          23       24       +1     
  Lines        2276     2359      +83     
  Branches      426      435       +9     
==========================================
+ Hits         2093     2150      +57     
- Misses        142      164      +22     
- Partials       41       45       +4

Impacted Files	Coverage Δ
sanic/app.py	92.43% <100%> (+0.16%)	⬆️
sanic/signals.py	62.74% <62.74%> (ø)
sanic/helpers.py	84.09% <68.18%> (-15.91%)	⬇️

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If this is incorrect, please respond with an update. Thank you for your contributions.

[ahopkins]

@harshanarayana, I would like to get this one rolling again so we can get it into a release soon. What is needed? I am happy to take it over if you cannot right now.

[ahopkins]

See #1902