Support for epoch winning bakers and first block queries.

[td202]

Purpose

Support GRPCv2 endpoints GetWinningBakersEpoch (for getting the list of bakers that won in a given epoch) and GetFirstBlockEpoch (for getting the first block in a given epoch).

Changes

• Add a new query result code that represents when a query refers to a future epoch.
• Implement getting the first finalized block of an epoch for both consensus versions.
• Implement the GetWinningBakersEpoch endpoint (supports consensus v1 only).
• Implement the GetFirstBlockEpoch endpoint (supports both consensus versions).

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[MilkywayPirate]

Looking good!
A few comments and questions.

[MilkywayPirate]

Please update changelog.

[abizjak]

As discussed in the office, it's a bit annoying that these two queries take a BlockHashInput (potentially) as argument but do not respond with the block hash that was used for the query.

But since they can also take an epoch number it is not always possible to return a block like it is for other queries that take just the BlockHashInput.

So it's probably the best if it stays like this.

And in the SDKs, we probably want to take only specific block hash, block height, or epoch number as arguments for the winning bakers to avoid mistakes.