add variable bound agent example

[joonjeong]

Description

Add variable-bound agent example with dead letter handling use case

[codecov-commenter]

Codecov Report

Base: 94.45% // Head: 94.45% // No change to project coverage 👍

Coverage data is based on head (831497a) compared to base (948dd05).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #388   +/-   ##
=======================================
  Coverage   94.45%   94.45%           
=======================================
  Files         102      102           
  Lines       11023    11023           
  Branches     1293     1293           
=======================================
  Hits        10412    10412           
  Misses        533      533           
  Partials       78       78           

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[wbarnha]

I'd like to test this out for myself before merging it, I have no issues merging this otherwise.

[joonjeong]

just run script is enough..!

faust $ python examples/bound_agent.py worker -l info

and you can see like this.