cli: add an alternate filename for config.yaml

Couple of our users have mentioned that `config.yaml` is too generic. We cannot remove the support for config.yaml, but we can introduce an optional new file `hasura-config.yaml` which can be selected by using a flag on the `init` command.

Later we could probably make it a default too.

The function which validates a folder to be a Hasura Project will need to be updated accordingly.