Add a getter for all active validationIDs

[cam-schultz]

Context and scope
Validator information (including validationIDs) are stored in a mapping in the validator manager contract's state. Mappings are not iterable, so there's no way to fetch a value from this mapping without already knowing the validationID. This is poor UX for any methods that use the validationID, since it requires that the caller know the validationID a priori through some means other than the validator manager contract.

A getter of the form getActiveValidators() public view returns ([]bytes32) would improve this UX.

Discussion and alternatives
One way to implement this is to define two new state variables:

1. An array bytes32 validationIDs
2. A mapping of bytes32 => (int index, bool status),validationIDIdx where the key is the validationID, and the value is a tuple of the validationID's index in (1), and it's status (true if active, false otherwise)

When a new validator is registered, it's validationID is appended to validationIDs, and validationIDIdx[validationID] is set to (len(validationIDs)-1, true)

When a validator is removed, validationIDIdx[validationID] is set to (validationIDIdx[validationID].first, false), acting as a tombstone.

Finally, when getActiveValidators is called, we iterate over validationIDs, populating the return array with elements whose corresponding status in validationIDIdx is true.

Gas cost
Validator information is stored in a mapping rather than an array in order to prevent common operations such as initiateValidatorRegistration from performing an O(n) iteration over the entire validator set. The approach described above would move that expensive operation to a view method that is not called by any core validator management functions.

Open questions

• This same principle can be applied to fetch active delegators from StakingManager.

[geoff-vball]

I think we can probably get away with only storing the index in validationIDIdx[validationID]. When we remove a validator, we can delete (or set to 0 which is equivalent) the entry in validationIDs. That way, in order to get all the active validators, we need only iterate over validationIDs and return any non-zero entries.

[0xJohnnyGault]

Why not use EnumerableSet from OZ? https://docs.openzeppelin.com/contracts/5.x/api/utils#EnumerableSet

[cam-schultz]

Why not use EnumerableSet from OZ? https://docs.openzeppelin.com/contracts/5.x/api/utils#EnumerableSet

This is a great suggestion, thanks. EnumerableSet implements @geoff-vball 's suggestion directly. IMO the main barrier to implementing this is how to track existing validators that aren't indexed in an enumerable way. We'll likely need some single-use migration code to accomplish that.