Missing possibility to set custom postMessage origin in the SPA SDK

Problem:
Currently, the SPA SDK is using origin from the API URL to verify and address communication with the Channel Manager. Whenever the Page Model API is served from a host other than CMS, the integration will not work.

Reproduction Path:
Try to access an SPA channel from `localhost:9080` using the HAP.

Solution:
Expose `origin` configuration parameter to provide possiblity to customize that option.