* feat: Add AWS instance identity authentication
This allows zero-trust authentication for all AWS instances.
Prior to this, AWS instances could be used by passing `CODER_TOKEN`
as an environment variable to the startup script. AWS explicitly
states that secrets should not be passed in startup scripts because
it's user-readable.
* feat: Support caching provisioner assets
This caches the Terraform binary, and Terraform plugins.
Eventually, it could cache other temporary files.
* chore: fix linter
Co-authored-by: Garrett <garrett@coder.com>
13cef7d07c