Constants Parameter Resolver

[patrobinson]

We find ourselves putting account IDs in parameter files a few times and this opens a potential enhancement.

We could have a Constants parameter resolver that works like so:

permitted_account:
  constant: account_id/my_awesome_other_account

Where this looks up another YAML file in constants/account_id.yml:

my_awesome_other_account: 0123456789012

[patrobinson]

The Latest AMI resolver also accepts an account ID as input, so there would also be benefit in supporting nested parameter resolvers, but this would be rather difficult to implement.

[simpsora]

I'd be interested to see how secrets could work here -- I can see a need to share a secret with multiple stacks, and since we already have secret support (via dotgpg), making it usable here would be nice.