Add Adzerk SDK version header

[karnowski]

Background

The four new Decision SDKs (Java, JavaScript, Ruby, & Python) transmit an X-Adzerk-Sdk-Version header to the ad server & UserDB server to let us know the SDK name and version. We need this telemetry from the iOS and Android SDKs too. This improves Adzerk's ability to debug customers' API use as well as plan improvements to the SDKs.

Acceptance Criteria

• The SDK should send an SDK header on all ad server and UserDB requests showing it's the iOS SDK and its package version.
• This header will use the X-Adzerk-Sdk-Version header.
• This header will have a payload of: [github-repo-name]:[package-version]
  • example: adzerk-ios-sdk:1.1

[karnowski]

Note that the version should change when we cut a new release.