Add CA bundle version and pinning status to user agent string

[mihir-pradhan]

Summary

• Added CA_BUNDLE_VERSION = '1.0' constant in duo_universal/client.py
• Modified exchange_authorization_code_for_2fa_result to append ca_bundle/1.0 and ca_pinning: enabled|disabled to the user agent string based on the disable_ca_pinning runtime state

How Has This Been Tested?

• test_user_agent_includes_ca_bundle_version — verifies ca_bundle/1.0 is present in the user agent
• test_user_agent_ca_pinning_enabled — verifies ca_pinning: enabled when pinning is not disabled
• test_user_agent_ca_pinning_disabled — verifies ca_pinning: disabled when disable_ca_pinning=True

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)