Add a view method in NOR to get a range of keys

[avsetsin]

The getSigningKey method is now used to fetch node operator keys, which can fetch them only one at a time, causing some complexity on the tooling side. I propose to add a method that takes operator_id, from_index and to_index as arguments and returns an array of keys

[rkolpakov]

It's now implemented at this commit.

[TheDZhon]

Resolved in https://github.com/lidofinance/lido-dao/releases/tag/v2.0.0