Expose validator set queries needed for Fibre / PFF verification

## Problem

Clients verifying `MsgPayForFibre` validator signatures need access to the validator set used for those signatures.

Today, this is not clearly exposed through the node API, but it is required for clients that want to verify PFFs or related Fibre attestations.

## Why this matters

To verify a PFF, a client needs more than the payment promise and signatures. It also needs the validator set for the relevant height in order to:
- identify the validators
- verify signatures against the correct public keys
- check voting power thresholds

Without a validator set query API, clients cannot perform PFF verification cleanly through `celestia-node`.

## Proposed work

Add node API support for querying validator sets at the relevant height so Fibre-aware clients can verify PFFs without needing separate consensus-specific plumbing.

## Acceptance criteria

- `celestia-node` exposes validator set queries suitable for PFF verification
- The API makes it clear which height should be queried
- The returned data is sufficient to verify validator signatures and threshold conditions
- Fibre clients can use this API directly as part of PFF verification flows