Allow setting the HTTP Origin header

[eugenedorfman-optable]

Motivation

Follow-up API addition requested after the Resolver Specific Parameters (ID5 Mobile In-App) work (#57). These changes should come after that release ships, and get their own release.

Mobile HTTP stacks do not send an Origin header by default. The Optable edge uses the request origin for several server-side features: some EID resolvers require it to be present, and it also feeds site attribution and origin validation. Allowing the app to set it enables those features for mobile SDK traffic.

Details

Allow the integrating app to set the HTTP Origin header value sent with Optable API requests.

Objective

1. Add an optional origin setting to the SDK configuration.
2. When set, include Origin: <value> on all Optable API requests (identify, targeting, profile, etc.). When unset, behavior is unchanged (no Origin header).
3. Unit test coverage for request forming with and without the configured origin.
4. Document the new option and show it in the demo app configuration.