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

[cisco-dmytro-hissa]

Description

• Added a CA_BUNDLE_VERSION constant (ca_bundle/1.0) to DuoApi.
• Added a private AppendCaInfo(string agent) helper that appends the CA bundle version and the current pinning status —(ca_pinning=enabled) or (ca_pinning=disabled) — to a User-Agent string.
• Set the request's UserAgent via AppendCaInfo(this.user_agent) so the pinning status reflects runtime state (e.g. after DisableCaPinning()).

How Has This Been Tested?

• TestDefaultUserAgent (updated): asserts the CA bundle version is present and the string ends with (ca_pinning=enabled).
• TestCustomUserAgent (updated): asserts the full expected User-Agent including the CA bundle version and (ca_pinning=enabled).
• TestUserAgentCaPinningDisabled (new): calls DisableCaPinning() and asserts the User-Agent ends with (ca_pinning=disabled).

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)