Request Subset of Secrets

[dsanders11]

Some jobs may only want a subset of secrets, would be nice to allow requesting a subset with query parameters. Since the default format does an export for each secret key, this can be problematic since it will clobber any existing environment variables with the same name. We ran into this on Fiddle where we still needed a secret from the secrets service, but also needed to provide a value for one of those keys in the environment (too slow to get Infra WG to update the secrets) and that ran into the clobbering situation.