Add CA bundle version and pinning status to the user agent string

[cisco-dmytro-hissa]

Description

• Added CA_BUNDLE_VERSION = '1.0' constant in duo_client/client.py
• Modified Client.__init__ to append ca_bundle/{version} and (ca_pinning={enabled|disabled}) to the user agent string based on the disable_ca_pinning runtime state
• Empty/falsy user agent strings are left unmodified

How Has This Been Tested?

• test_default_user_agent_includes_ca_bundle_version — verifies ca_bundle/1.0 is present in the default user agent
• test_default_user_agent_includes_ca_pinning_enabled — verifies (ca_pinning=enabled) when pinning is not disabled
• test_user_agent_includes_ca_pinning_disabled — verifies (ca_pinning=disabled) when disable_ca_pinning=True
• test_default_user_agent_format — asserts the full expected user agent string format
• test_custom_user_agent_includes_ca_fields — verifies CA fields are appended to custom user agent strings
• test_empty_user_agent_not_modified — verifies an empty user agent is not modified

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)