Add a method to the Validator Manager so as to obtain the list of all the validators in the subnet

[felipemadero]

Context and scope
There is no way to obtain such a list from the contract itself.

Discussion and alternatives
Include a description of the changes to be made to the code along with alternatives
that were considered, including pro/con analysis where relevant.

Open questions
Questions that are still being discussed.

[cam-schultz]

Is there any reason why GetValidatorsAt can't be used instead? This is what we use in our E2E tests to construct Warp signatures.

Exposing this in the contract would require us to maintain additional state in the contract, since there's no way to extract all set keys from a mapping in Solidity.