[Feature] Full Validator list endpoint

[TarikGul]

/pallets/staking/validators

Description

Should return a list of all the validators. This requires retrieving all the entries and mapping out the AccountId keys from storage.

const validators = await api.query.staking.validators.entries();
const keys = validators.forEach(([key]) => {
  console.log('key arguments:', key.args.map((k) => k.toHuman()));
});

Example JSON response from the endpoint:

{
    validators: [
        `162DmeD1MHpWQhv8NPz7zbUEfCyTfG4Tuqz7P7852KiY2wXz`,
        `13pYWKctR5s8vQuyZt3pxQXue4SRH9coyAS9S9z5HtogAnhs `,
        ...
    ]
}

[Imod7]

Another related endpoint to the one mentioned in this issue and maybe interesting to have is one that shows
"The list of nominators per validator"

For example under an endpoint like :
/pallets/staking/validators/validatorAddress

This idea comes from seeing related questions in the Substrate Stack Exchange like the ones mentioned below :

• How to find all nominators of a validator?
• How to query the active nominators of X validator in past eras?
• How to get the nominators for a given validator address?