feat: add solution_channel parameter to API loader

[wangela]

There is a new parameter in Maps Platform JS and WS APIs [solution_channel](https://developers.google.com/maps/documentation/javascript/url-params#optional_parameters).

The generator(s) in /shared could be updated to take an optional value for a property solution_channel and append it to the loader URL in the same way that callback and version are supported in the /samples/store-locator/package.json file. I think /shared/layout.njk L63 is where these parameters are loaded and the support for the new parameter needs to be added.