Apikey header name

Currently, we set the both headers - `Authorization` and `X-CKAN-API-Key`.

Some other plugins use the `ckanapi` for the `RemoteCKAN` client. Like the [ckanext-syndicate](https://github.com/DataShades/ckanext-syndicate), for example. And in the situation, when the remote portal is protected with the "Basic" HTTP authentication, we can't pass both basic auth authorization and ckan apikey.

Also, because we have [this ](https://docs.ckan.org/en/latest/maintaining/configuration.html#apitoken-header-name)config option, the portal can use any header name for API token, and that's why we should make it configurable.