add oxide env subcommand to populate environment with OXIDE_HOST and OXIDE_TOKEN variables

[ahl]

I've found something like this (https://github.com/transposit/awscli-as-session) to be useful for running terraform e.g. oxide env terraform plan would run terraform with OXIDE_HOST and OXIDE_TOKEN set in that environment. This will further be useful when Oxide supports MFA, and keys that expire.

[karencfv]

ooooh this is nice!

I was also thinking that when it's possible to set a default host on the CLI's hosts.toml file, the terraform provider could pull in the the information from the default host and use that as authentication information.