Add PipelineTag helper

[fafhrd91]

Introduce PipelineTag, which allows to notify pipeline binding

[codecov]

Codecov Report

Attention: Patch coverage is 86.79245% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.06%. Comparing base (cb0c03f) to head (50a5654).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ntex-service/src/ctx.rs	85.41%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #441   +/-   ##
=======================================
  Coverage   91.06%   91.06%           
=======================================
  Files         200      200           
  Lines       32205    32238   +33     
=======================================
+ Hits        29326    29359   +33     
  Misses       2879     2879           

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