feat: Add support for custom HTTP requests

[SMadani]

This PR allows users of the SDK to declaratively make custom requests with their own JSON-based request and response types and to specify the URL and HTTP method, whilst automatically handling the authentication, serialisation and deserialisation. This is implemented as methods on the new CustomClient class, accessible from VonageClient#getCustomClient().

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.87%. Comparing base (281a3b3) to head (a60e8de).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #581   +/-   ##
=========================================
  Coverage     99.87%   99.87%           
- Complexity     3395     3412   +17     
=========================================
  Files           515      516    +1     
  Lines          7320     7347   +27     
  Branches        403      406    +3     
=========================================
+ Hits           7311     7338   +27     
  Misses            3        3           
  Partials          6        6           

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