[Feature]: dynamic signing scheme config with BLS support

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the feature

## Summary
We want pluggable signing schemes, starting with BLS12-381 alongside secp256k1. Add a config flag to pick the scheme at runtime. Update the validator manager contract so it accepts unbounded `bytes` pubkeys (no fixed-length secp checks).

## Scope
- Add config option for signing scheme and wire it into node startup.
- Implement a BLS signing provider using the existing provider trait.
- Generalize public key handling across types and utils.
- Update `ValidatorManager.sol` to take unbounded `bytes` keys and derive validator IDs from `keccak256(publicKey)`.
- Update CLI/tools to parse, generate, and show BLS keys.

## Changes Needed
- **Contract refactor**: [`solidity/src/ValidatorManager.sol`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/solidity/src/ValidatorManager.sol) moves from secp256k1 structs/parsing to unbounded `bytes` public keys + generic validation + address derivation from raw bytes.
- **Signing provider**: new BLS provider implementation aligned with existing signing provider traits in [`types/src/signing/`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/types/src/signing/).
- **Config + runtime wiring**: add `signing_scheme` config in [`cli/src/config.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/cli/src/config.rs) and select provider in [`app/src/node.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/app/src/node.rs) / [`app/src/state.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/app/src/state.rs).
- **Key handling across the stack**: update Rust types + utilities to accept variable-length keys and BLS formats: [`types/src/context.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/types/src/context.rs), [`types/src/validator_set.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/types/src/validator_set.rs), [`utils/src/poa.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/utils/src/poa.rs), [`utils/src/genesis.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/utils/src/genesis.rs), [`utils/src/validator_manager/types.rs`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/utils/src/validator_manager/types.rs), and CLI commands under [`cli/src/cmd/`](https://github.com/informalsystems/emerald/blob/145f116e5801fc78086955de01cd7a41b138c12b/cli/src/cmd/).

## Acceptance Criteria
- 48‑byte BLS pubkeys register + read fine in `ValidatorManager.sol`.
- Node runs with `signing_scheme = "bls12381"` and signs/verifies votes/proposals.
- secp256k1 remains default and still works.
- CLI/tools accept and output BLS keys without format errors.

## Remark
- No BLS signature aggregation in this change. Just swap in BLS as a scheme. We can revisit aggregation later.
- BLS support unlocks succinct signatures, which could be useful in remote contexts (e.g. bridge flows).