Add passing optable IDs example for Prebid SDK into the demo app

[ValentinPostindustria]

Original issue.

Motivation

We want to demonstrate how to use the Optable SDK alongside the Prebid SDK so that publishers can easily integrate Optable into their existing Prebid-based implementations.

Implementation

• Integrate the Prebid SDK into both the Java and Kotlin demo apps.
• Add a banner example that uses the Optable SDK together with the Prebid SDK (Original API).
• Pass user identifiers to the SDKs using the latest External User IDs API.
• Provide comments to sample code for both Java and Kotlin.