feat: add custom queries for calculating validators specific epoch reward

[jeongseup]

Describe your changes

Add some custom rpc queries for extracting onchain data for validators and delegators.

1. First one is for getting first block height by epoch

    // First block height of the current epoch
    ( "first_block_height_by_epoch" / [epoch: Epoch] ) -> Option<BlockHeight> = first_block_height_by_epoch,

2. Second one is for getting validators' reward products by epoch.

    ( "rewards_products" / [validator: Address] / [epoch: opt Epoch])
    -> Vec<(Epoch, Dec)> = rewards_products,

Actually, some of validators like us want to make specific epoch reward for validators and delegator. At that time, both of queries will be very useful. If your team want, I can attach calculating logic with that.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[jeongseup]

@tzemanovic Could you please review this PR?