Common implementation for JWT based heartbeat auth

[DavidS-ovm]

• based on the work in Common CLI and config implementation for engine hosts #360, add support for SOURCE_ACCESS_TOKEN (string), SOURCE_TOKEN_TYPE (string), and OVERMIND_MANAGED_SOURCE (bool, hidden) with appropriate descriptions to receive the config from https://github.com/overmindtech/srcman/issues/296
• extend the EngineConfigFromViper method to create a natsTokenClient like https://github.com/overmindtech/aws-source/blob/7788d00d18d2394548f92969e98ffe2db09d4621/cmd/root.go#L128-L153 does and configure that as HeartbeatOptions.ManagementClient. Using either the api key as already implement or the provided access token in a static token source (see sdp-go/auth/auth.go#NewOAuthTokenClientWithContext).
• update and deploy aws-source with this version for testing
• tag new version of discovery library
• update all sources to the new version and clean up engine setup code to use the new way
  • aws-source
  • k8s-source
  • stdlib-source
• deploy all of the above to dogfood and test authentication using this method
  • https://ui.honeycomb.io/overmind/environments/test/datasets/aws-source/result/5KNcLd3CD1P should show calls from the managed aws-sources in the dogfood EKS cluster to the df api-server
• deploy to prod