juju client doesn't pass version and other useful metadata to api calls

[Gui Bot]

When the juju client makes api calls it should report the client version as part of any request. I'd also suggest we start adding the model UUID as part of those as well.

This is specifically to help support the charmstore aiding in making things seamless across versions of Juju where a feature might not be supported in the past. In particular the desire was to cheat the min-juju-version by allowing charms to add support and gating charm access in the store based on client version.

This should be passed as a header to any endpoint so that we have the ability on providing intelligent errors, or tweaking functionality to be more helpful as clients move from version to version.

[Heather Lanigan (hmlanigan)]

We've taken a different approach and the client can ask for the best api version of a facade to make calls. Each facade version can have different api calls for different behavior.