Add SKOverlay Support

[BaiShuoyang]

Is your feature request related to a problem? Please describe.
In-app publishers have seen performance increases when this SKOverlay is available as it shows a more interactive module during rendering. To marketers and DSPs, presenting this overlay automatically registers a preferred attribution signal (fidelity 1) to Apple for determining a winner of a download. DSPs are more likely to rewarded by running SKOverlay campaigns, hence more budget for publishers.

Describe the solution you'd like

1. Prebid Mobile SDK to support rendering the SKOverlay on IOS inventory.
2. Prebid Mobile SDK supports SKOverlay controls set by DSPs such as video delay (Time duration after the video shows the overlay), position of the overlay, whether the SKOverlay can be dismissed by the user, etc. This will requires the Prebid Server to support passing and receiving SKOverlay control signals in the bid request and responses with the PBS bidders.

Reference

• Applovin: https://dash.applovin.com/documentation/mediation/auctions/dsps/integration
• Inmobi: https://support.inmobi.com/monetize/api-integration-guidelines/bid-responses-dsp/#object--bid
• Chartboost: https://partners.chartboost.com/#skoverlay-support
• Digital Turbine: https://developer.digitalturbine.com/hc/en-us/articles/8938168894621-SKOverlay
• IAB SKAdNetwork Extension Improvements: SKOverlay and Event Support: https://docs.google.com/document/d/1kXkw5XHsztZsaMjL9MDDvVkkiaOykreAotTTXfSpMqg/edit#heading=h.vbouqzt75g27)

[alexsavelyev]

@bretg can you please check with the Prebid Server team if the bidders can already pass the SKOverlay related fields if they would build it in the adapters? Would we need to update a documentation for it?

[bretg]

Adapters can supply arbitrary response extensions for $.seatbid.bid.ext with the exception of origbidcpm, origbidcur, and prebid which may be overwritten by Prebid Server Core. They can supply these fields in the BidderResponse.Bids[i].Bid.Ext

[jsligh]

Steps for implementation:

1. Signal SKOverlay (SKAN support from 1.0->4.0)
2. Render SKOverlay (Auto Store)
3. Trigger StoreKit

Useful links:
https://developer.apple.com/documentation/storekit/skoverlay
https://developer.digitalturbine.com/hc/en-us/articles/11846217613085-Auto-Store
https://support.vungle.com/hc/en-us/articles/360056673092-The-Liftoff-Monetize-Conversion-Experience#app-store-0-3

[jsligh]

Thoughts on integration: https://docs.google.com/document/d/1zsqqrI_tuEsl9pxUzoeBe-a6WRTcuSyd3DVNOIG0bIw/edit?usp=sharing

[jsligh]

Related to #997