Force TUNNEL proxy type in MQTT5 client

[sfod]

Issue #, if available: IOTSDKT-469

The downstream projects pass unsupported proxy connection type by default. Currently, MQTT5 client initialization fails when this happens.

Description of changes:

Since the only proxy connection type that can be supported is TUNNEL, force this type.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.69%. Comparing base (df8ef58) to head (9423ece).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #399      +/-   ##
==========================================
- Coverage   84.71%   84.69%   -0.02%     
==========================================
  Files          25       25              
  Lines       10490    10490              
==========================================
- Hits         8887     8885       -2     
- Misses       1603     1605       +2     

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