Add appendToUserAgent() method to Client class

[jeffreyparker]

Summary

• Adds appendToUserAgent(string $extension) to Client, allowing callers to append custom identifiers to the User-Agent header sent with API requests
• Matches the pattern established in duo_universal_php for consistency across Duo PHP SDKs
• Uses a fluent interface (return $this) consistent with setRequesterOption() and disableCaPinning()

Design notes

• Last-call-wins semantics: Subsequent calls to appendToUserAgent() overwrite the previous value rather than concatenating. This keeps the UA string predictable and matches the duo_universal_php behavior.
• Trimming: The extension is trimmed before storage. If the result is empty/whitespace-only, nothing is appended (no trailing space in the header).
• Private property: $user_agent_extension is private rather than public to encourage use of the setter method.

Test plan

• Normal extension is appended after the base UA string
• Whitespace-only input results in no extension appended
• Leading/trailing whitespace is trimmed
• Multiple calls overwrite (last value wins)
• All 34 existing tests continue to pass