Allow custom user agent specification

[ysyrota]

Description

Added support for extension of user agent strings. Users can now extend a user agent via appendToUserAgent() method of Client class.

Motivation and Context

This change was specifically requested to allow duo_universal_wordpress (the WordPress plugin) to specify its own user agent when making requests to Duo's servers.

How Has This Been Tested?

• All existing tests pass
• New test coverage added:
  • testAppendToUserAgent() - verifies that the user agent extension can be set and is included in requests
  • testUserAgentWithoutExtension() - verifies that user agent works correctly without any extension
  • testAppendToUserAgentEmpty() - verifies that empty user agent extension is handled correctly
• Manual testing with scripts using both default and custom user agent functionality

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)