✨ add final class modifier to generated Chopper API implementation

[techouse]

This PR adds the final class modifier to the generated Chopper API implementation, i.e.

before

class _$MyService extends MyService {
  _$MyService([ChopperClient? client]) {
    if (client == null) return;
    this.client = client;
  }
  
  // ... stuff
}

after

final class _$MyService extends MyService {
  _$MyService([ChopperClient? client]) {
    if (client == null) return;
    this.client = client;
  }
  
  // ... stuff
}

[codecov]

Codecov Report

Merging #463 (c6774e6) into develop (7a6b5b9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #463   +/-   ##
========================================
  Coverage    93.70%   93.70%           
========================================
  Files            8        8           
  Lines          445      445           
========================================
  Hits           417      417           
  Misses          28       28           

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