Please add an interface to firefly_client for supplying authorization headers to be added to the requests and uploads transmitted to the Firefly server.

At first this should just be an API for "manually" supplying a token obtained elsewhere.

At a later date it may be appropriate to have an option for firefly_client to pick up a token, if available, from the environment in which it is running, e.g., from an environment variable or a dot-file, but that decision is yet to be taken.

For now the token, for use in the LSST environment, will be obtained from the API Aspect's Get-Token web page, as recently prototyped by Brian Van Klaveren.