feat: add /pallets/staking/validators endpoint

[Imod7]

Relates to this Issue #1005

Description

Adds /pallets/staking/validators endpoint which returns a list of all validators addresses and their corresponding status which can be either active or waiting.

• Active means that the validator is part of the active set
• Waiting means that the validator did not get into the active set this era

Sample Response

Sample request :
/pallets/staking/validators

Sample response :

{
  "validators": [
    {
      "address": "HYxzaXwkydDNAy3R8UnCvfprNm74TwwSELpyXSRoqmy9AQY",
      "status": "waiting"
    },
    {
      "address": "DfishveZoxSRNRb8FtyS7ignbw6cr32eCY2w6ctLDRM1NQz",
      "status": "active"
    },
    {
      "address": "HmsTAS1bCtZc9FSq9nqJzZCEkhhSygtXj9TDxNgEWTHnpyQ",
      "status": "active"
    },
    {
      "address": "Efishjj6mrTWVjwFoxtMmHH3mX6tRNnPKsSfQeCLZSv3jAE",
      "status": "waiting"
    },
    {
      "address": "Hn8ELsyUL9UgDZpPX6bE464o4sbi7gqgYEKjntK7xuVzGXH",
      "status": "active"
    },
...

Todos

• Update Docs
• Add Tests

[TarikGul]

closes: #301

[TarikGul]

Looks great so far, just some nits, and code changes.

[Imod7]

@TarikGul @IkerAlus If validators can change status (go from active to waiting for example) per era then I have to add as param in the endpoint also the era right ? Right now I only have the block height.
From Polkadot Support

An era is a period of time during which there is a specific set of active validators. Each era has 6 epochs (or sessions). During the last epoch, the active set of the next era is elected.

[IkerAlus]

@Imod7 Either you add blockHeight or era as input parameter but not both. There is an unequivocally relationship between a block and an era.

If the question is what is more useful as input parameter, block heights or eras, I am not sure, I see reasons for both:

• block height is used everywhere in the endpoints and it is easy to find
• The concept of era is more related to this endpoint

[TarikGul]

Just one nit. Great Job! lgtm

[marshacb]

looks great to me! once merge conflicts and any remaining nits are addressed its good to go!