feat: Add support for proxies in HttpConfig

[SMadani]

This PR allows users to set a proxy URL without needing to use the deprecated VonageClient.Builder#httpClient method. This can instead be configured directly in HttpConfig without exposing details of the underlying HTTP client implementation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.00%. Comparing base (fe06b81) to head (09447d7).
Report is 72 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #556      +/-   ##
============================================
+ Coverage     95.60%   96.00%   +0.39%     
- Complexity     3427     3754     +327     
============================================
  Files           513      564      +51     
  Lines          8141     8704     +563     
  Branches        451      497      +46     
============================================
+ Hits           7783     8356     +573     
+ Misses          198      175      -23     
- Partials        160      173      +13     

Files with missing lines	Coverage Δ
...rc/main/java/com/vonage/client/AbstractMethod.java	100.00% <100.00%> (+22.22%)	⬆️
src/main/java/com/vonage/client/HttpConfig.java	93.93% <100.00%> (+21.59%)	⬆️
src/main/java/com/vonage/client/HttpWrapper.java	100.00% <100.00%> (ø)

... and 32 files with indirect coverage changes