feat: add solution_channel parameter to API loader

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](https://github.com/googlemaps/js-samples/blob/main/samples/store-locator/package.json) file. I think [/shared/layout.njk L63](https://github.com/googlemaps/js-samples/blob/main/shared/layout.njk#L63) is where these parameters are loaded and the support for the new parameter needs to be added.