chore: remove subscriber

[achettyiitr]

Description

• Since (cp *ControlPlane) Subscribe() return something, it need to be exported. Currently, it's returning *cache.Subscriber which is internal.
• Since the Subscriber struct, currently does not add much of a value, removing it.

Linear Ticket

• Resolves PIPE-715

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (18b146c) 97.74% compared to head (2da0dec) 97.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   97.74%   97.67%   -0.07%     
==========================================
  Files           7        6       -1     
  Lines         177      172       -5     
==========================================
- Hits          173      168       -5     
  Misses          4        4              

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