R1:f18ee5c237fe

Generate and use "cluster" Conduit API tokens

Summary:
Ref T5955. Ref T2783.

- Removes the "temporary" type. I was going to use this for T3628 but it started taking more time than I wanted to spend on it.
- Add a "cluster" type, which is an internal-only token type used within a cluster. This token value is never shown to the user.
- Automatically generate, use, and cycle cluster tokens.

Test Plan:
- Diffusion (mostly) works with a repository configured to use a remote service.
- Saw cluster…