feat: customConsentGdpr integration

[cimperator]

Description

Add support for programmatic custom vendor consent (GDPR) to sourcepoint_unified_cmp, using the same native consent instance as loadMessage / loadPrivacyManager.

Requirements
Implementations of the following functions:

Future<SPConsent> customConsentGdpr({
  required List<String> vendors,
  required List<String> categories,
  required List<String> legIntCategories,
});
 

Optional (symmetry with native SDK):

Swift
Future<SPConsent> deleteCustomConsentGdpr({
  required List<String> vendors,
  required List<String> categories,
  required List<String> legIntCategories,
});

On success:
Persist consent in native storage.
Update the registered ConsentChangeNotifier (updateConsent).
Invoke existing delegate callbacks (onConsentReady and/or onSpFinished) with the updated SPConsent.

Native APIs to integrate
Android — com.sourcepoint.cmplibrary.SpConsentLib (same instance as in SourcepointUnifiedCmpPlugin):

spConsentLib.customConsentGDPR(
    vendors = listOf("..."),
    categories = listOf("..."),
    legIntCategories = emptyList(),
    success = { spConsents: SPConsents? -> /* map to HostAPISPConsent, notify Flutter */ },
)

Docs: android-cmp-app — Adding or Removing custom consents.

iOS — SPConsentManager in ConsentViewController (same instance as in SourcepointUnifiedCmpPlugin):

consentManager.customConsentGDPR(
    vendors: ["..."],
    categories: ["..."],
    legIntCategories: [],
) { /* handler */ }

Docs: ios-cmp-app — Adding or Removing custom consents.

[thekorn]

@cimperator there is an implementation as i understood this feature in #333 - does it fit your needs?