Attestations circuit: constraint attestation data variables

To allow exporting or adding equality constraints for fields of the `AttestationData`:
```rust
pub struct AttestationData {
pub slot: u64, // not used currently, might be used to check that attestations are for unique slots
pub index: usize, // == id of the committee in validators table
pub beacon_block_root: [u8; 32], // not used currently
pub source: Checkpoint, // not used currently, might be used to connect previous proof recursively
pub target: Checkpoint,
// target.epoch == target_epoch in ValidatorsCircuit
// target.root == state_root in StateCircuit
}
```

### Note:
- By specifying the `is_rlc` boolean in [`HashInputChunk`](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/witness/hashing.rs#L131) you can make Sha256Circuit to either calculate RLC encoding for input bytes or value (both on per chunk basis)
- The `implementations` of `From` determine `is_rlc` value automatically such `is_rlc: bytes >= 32`
- Based on `is_rlc` `SHA256Table.input_chunks` would either be [RLC or value](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/sha256_circuit.rs#L973) and Sha256Circuit would select [corresponding](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/sha256_circuit.rs#L579-L587) gates
- The final value/RLC of `input_chunks` can be found in the row with `is_enabled == 1`

### Scope
- Include `input_chunks` into [`AssignedHashResult`](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/gadget/crypto/sha256.rs#L41) by importing it's final value in halo2lib `AssignedValue` form in [`Sha256Chip`](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/gadget/crypto/sha256.rs#L48)
- See example of such importing [here](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/sha256_circuit.rs#L991-L992) and [here](https://github.com/ChainSafe/banshee-zk/blob/cc0991fd640d8402902de760508b7b4b185484af/zkcasper-circuits/src/gadget/crypto/sha256.rs#L145-L146) - this example demonstrates import of `Sha256Table.input_rlc`
- After value/rlc is present in `AssignedHashResult` you can use these values to perform equality checks
- To be able to do permutation checks in halo2lib (using `region.constraint_equal`) you would likely need to include `AssignedCell` along with `AssignedValue`.
- For the purpose of this task, only add constraints on inputs that are used in other circuits (see comment).