Provide a possibility to set a custom prefix for the Page Model API links

Use case:
When the CMS is running in the intranet behind some reverse proxy. In this case, an application requests the Page Model via some custom reverse proxy host. The client-side navigation on the SPA side requires some link rewriting mechanism to get accessible from the outside API links.

Requirements:

• Introduce an option (e.g., `apiHost`) behaving similarly to `cdnHost`.
• Replace host in the Page Model API link with the value from the option as-is.

Example:
In the following example, we set `hst:apiHost` to `https://api.example.com/brxm`.

Before:

"_links": {
  "self": {
    "href": "http://127.0.0.1:9080/site/_cmsinternal/spa-react-csr/resourceapi/news/"
  }
}

After:

"_links": {
  "self": {
    "href": "https://api.example.com/brxm/site/_cmsinternal/spa-react-csr/resourceapi/news/",
    "type": "external"
  }
}