This configures the user agent with the bricks version and the name of
the command being executed.
Example user agent value:
```
> * User-Agent: bricks/0.0.21-devel databricks-sdk-go/0.2.0 go/1.19.4 os/darwin cmd/sync auth/pat
```
This is a follow up for #194.
We intend to let non-bundle commands use bundle configuration for their
operating context (workspace, auth, default cluster, etc).
As such, all commands must first try to load a bundle configuration.
If there is no bundle they can fall back on taking their operating
context from command line flags and the environment.
This is on top of #180.
Not settled whether this should live as a top level command or hidden
under some debug scope. Either way, the ability to make arbitrary API
calls and leverage unified auth is a super useful tool.
Pieter Noordhuis
Serge Smertin