AWS SDKs universally support the ECS Credential Service, which supports
reading the service URI from environment variables.

Providing an http interface allows the subprocess to refresh credentials
as long as the master credentials session is valid.

Supporting the ECS Credential provider offers the following advantages
over the EC2 Metadata provider:

- Binding to a random, ephimeral port
  - Does not require adminstrator privileges
  - Allows multiple providers simultaneously for discrete processes
  - Partially mitigates the security issues that accompany the EC2
    Metadata Service because the address is not well-known
- Requiring an Authorization token further mitigates the potential for
  another process to access the credentials, since the Authorization
  token is only exposed to the subprocess via environment variables