Support environment-specific credentials

[begelundmuller]

We currently support different environments (specified with rill start --env NAME, defaults to dev on local and prod in cloud).

We should support optionally specifying variables and connector credentials on a per-environment basis.

We can easily support that by introducing a convention that variables with a [env_name] prefix are environment specific. For example:

# Applies to all environments
connector.s3.aws_access_key_id=...

# Overrides the variable used in prod
[prod]connector.s3.aws_access_key_id=...

Task list:

• [ ] Add the env prefix to new variables created with rill env configure when --env NAME is passed
• [ ] Parse the env prefix in the runtime when resolving variables
• [ ] The rill env pull command should omit variables marked for the prod environment