Update Prebid Mobile SDK documentation to reference 3rd identity vendors

[alexsavelyev]

User Story 1:

At the page https://docs.prebid.org/prebid-mobile/pbm-api/android/pbm-targeting-params-android.html#user-identity, please add a new section:

“External References”

Certain identity vendors require an external dependency to generate user identity and then to pass it via Prebid SDK. Please note that these are references to a 3rd party code and Prebid has not inspected it. Links to the documentations of those references will be listed in this section.

UID SDK:

• Github repo: https://github.com/IABTechLab/uid2-android-sdk
• Documentation: https://unifiedid.com/docs/guides/integration-prebid-mobile-summary. There is a helper lib for Android Prebid Mobile SDK integration https://unifiedid.com/docs/guides/integration-mobile-client-side#optional-uid2-prebid-mobile-sdk-integration.

User Story 2:

At the page https://docs.prebid.org/prebid-mobile/pbm-api/ios/pbm-targeting-ios.html#user-identity, please add a new section:

“External References”

Certain identity vendors require an external dependency to generate user identity and then to pass it via Prebid SDK. Please note that these are references to a 3rd party code and Prebid has not inspected it. Links to the documentations of those references will be listed in this section.

UID SDK:

• Github repo: https://github.com/IABTechLab/uid2-ios-sdk
• Publisher needs to manually obtain the UID2 token using UID2Manager.shared.getAdvertisingToken() (refer https://unifiedid.com/docs/guides/integration-mobile-client-side#pass-generated-token-for-bidstream-use) from UID2 iOS SDK and pass it to Prebid Mobile SDK via setExternalUserIds manually