AggLayer: create metadata from token information

[mmagician]

Currently we set METADATA_HASH to 0s, but the proper construction of this requires that we encode token information (name, symbol, decimals) and hash that with keccak:

/**
 * @notice Returns the encoded token metadata
 * @param token Address of the token
 */

function getTokenMetadata(
    address token
) public view returns (bytes memory) {
    return
        abi.encode(
            _safeName(token),
            _safeSymbol(token),
            _safeDecimals(token)
        );
}

Also, handling the metadata for bridging of the gas token is different: https://github.com/agglayer/agglayer-contracts/blob/60d06fc3224792ce55dc2690d66b6719a73398e7/contracts/v2/PolygonZkEVMBridgeV2.sol#L335