Introduce 1.12.2 version with changes for the original codebase

[YuriyVelichkoPI]

Motivation

Several feature requests for the original API were implemented, during the development of 1.14-beta. They are needed for the current users of the original API so we should release them.

Objectives

• Prepare the new branch release-1.12 from the tag 1.12.1
• Prepare the new branch release-1.12.2 from the branch release-1.12
• Introduce the next changes in release-1.12.2 (do not squash them on the release branch)
  • Pass app:content:url property into bid request #274
  • Fixed timeout problem #326
  • Introduced the possibility to add custom headers to prebid post requests. #338
  • Support OpenRTB app:content object in bid request #314
  • Handled response code 204 (BLANK), now returns NO_BID. #320
  • Support for Seller-Defined Audiences for First Party Data #358
• Update the SDK version to 1.12.2
• Prepare the podspec for release.