Improve java sdk consistency

[chaixdev]

• Centralized Error Handling: Added apiError(HttpResponse<?>) in TwikeyClient to handle both ApiError and apierror headers. This fixed an inconsistency between gateways.
• Robust Null/Blank Handling: Created RequestUtils.putIfNotNull with centralized logic to filter out both null and isBlank() strings. This standardized behavior across all request classes and mitigates a potential bug where blank values were not considered.
• Refactored PaylinkGateway: Brought PaylinkGateway in line with other gateways by introducing PaylinkRequest and PaylinkResponse models. The old method is preserved but @deprecated.
• Modernized Authentication: Migrated getSessionToken() from legacy HttpURLConnection to the modern HttpClient.
• Hmac comparison: replaced manual for loop comparison with more secure MessageDigest.equals()