feat(consensus): Replace static ValidatorSet with dynamic L2 contract-based validator fetching

[t00ts]

This PR replaces the static ValidatorSet used in consensus with a dynamic validator fetching system that can retrieve validators from an L2 contract at runtime. This enables the consensus system to work with validator sets that change over time based on on-chain state.

New L2ValidatorSetProvider implements the ValidatorSetProvider trait to fetch validators dynamically.

To keep things working with our current setup, it supports two modes:

• Config-based: When validator_addresses is provided in config, it will use those addresses with random keys.
• Contract-based: When no addresses are configured, it will fetch from L2 contract using validator_fetcher::get_validators_at_height()

The validator fetching logic is temporarily in a new crate validator-fetcher. This was done because I've also added an RPC method for us to conveniently test this whenever the spec is finalized. As mentioned in the TODO comments, the validator-fetcher crate and its RPC method will be removed once the final specification is available, and the functionality will be migrated directly into the consensus module of the pathfinder crate.

As a side note, I also updated our pathfinder_consensus::ValidatorSetProvider trait to allow for errors.

Here is the Cairo test contract I'm using in case you're curious, which btw is deployed here.

use starknet::ContractAddress;
use core::array::{Array, ArrayTrait};

/// Validator information structure
#[derive(Drop, Serde, starknet::Store)]
struct ValidatorInfo {
    address: ContractAddress,
    public_key: felt252,
    voting_power: u64,
}

/// Mock contract that returns validators at a given height
#[starknet::contract]
mod ConsensusValidators {
    use super::*;

    #[storage]
    struct Storage {}

    // Ed25519 public keys generated for testing (felt252-compatible)
    const VALIDATOR_1: ValidatorInfo = ValidatorInfo {
        address: 0x111.try_into().unwrap(),
        public_key: 0x3b2223a9c54386b0b4f0f6b58fc2ec1e3b7f21581397ee3c7f,
        voting_power: 1000,
    };
    const VALIDATOR_2: ValidatorInfo = ValidatorInfo {
        address: 0x222.try_into().unwrap(),
        public_key: 0xd7db502088c25c78f0dd01492f86878da27b8fd391b001ed,
        voting_power: 1500,
    };
    const VALIDATOR_3: ValidatorInfo = ValidatorInfo {
        address: 0x333.try_into().unwrap(),
        public_key: 0x43cbc327a0b331fd17e915196ac339321ffd3bcaa7308f49d2,
        voting_power: 2000,
    };
    const VALIDATOR_4: ValidatorInfo = ValidatorInfo {
        address: 0x444.try_into().unwrap(),
        public_key: 0x8474f2adcef8935233876183dce7bb9a9136f53c57e578a85c,
        voting_power: 1200,
    };

    /// Builds the rotated list for a given height.
    fn rotate(height: u64) -> Array<ValidatorInfo> {
        let mut out = ArrayTrait::new();

        let n: u64 = 4_u64;
        let start: u64 = height % n;

        let mut i: u64 = 0_u64;
        while i < n {
            let idx = (start + i) % n;
            match idx {
                0_u64 => out.append(VALIDATOR_1),
                1_u64 => out.append(VALIDATOR_2),
                2_u64 => out.append(VALIDATOR_3),
                _     => out.append(VALIDATOR_4),
            };
            i += 1_u64;
        }

        out
    }

    /// Returns validator addresses at a given height
    #[external(v0)]
    fn get_validators_at_height(self: @ContractState, height: u64) -> Array<ContractAddress> {
        let validators = rotate(height);
        let mut addresses = ArrayTrait::new();
        let span = validators.span();
        let mut i = 0;
        while i < span.len() {
            addresses.append(*span.at(i).address);
            i += 1;
        }
        addresses
    }

    /// Returns detailed validator information at a given height.
    #[external(v0)]
    fn get_validator_info_at_height(self: @ContractState, height: u64) -> Array<ValidatorInfo> {
        rotate(height)
    }
}

[t00ts]

See also: #2935

[CHr15F0x]

LGTM!