Support configuring and overriding account and workspace

[Jonathan Yu]

Users need a way to:

• Define the account (organization) that we are operating on, at the provider level
• Define the workspace that we are operating on, at the provider level

We expect that the most common use cases for this provider are:

• To manage service account keys corresponding to an account
• To manage resources in one or more workspaces (work pools, variables, etc)

It would also be convenient to allow people to override the provider-level defaults, so that we can support more comprehensive end-to-end scenarios, such as a configuration that creates a workspace and provisions other resources into that workspace (such as variables or work pools). We can also reuse this capability in automated tests, since we can set up and tear down a temporary workspace during our build.