Interpolate env in registry_auth

[r4victor]

Closes #1539

This PR makes the env context available in registry_auth of run configurations via ${{ env.ENV_NAME }} syntax. This allows passing registry_auth without specifying registry creds in configuration files:

type: task
image: my_registry/private_image
env:
  - REGISTRY_USER
  - REGISTRY_PASSWORD
registry_auth:
  username: ${{ env.REGISTRY_USER }}
  password: ${{ env.REGISTRY_PASSWORD }}

The interpolation is implemented on the client side. The support for specifying sensitive values without interpolation is to be addressed by #708.