Support whitelisting ZRC20 assets in bulk

[CharlieMc0]

Describe the Issue

We can only submit one /zetachain.zetacore.crosschain.MsgWhitelistERC20 message per group proposal which makes the whitelisting process much more time consuming.

Expected Outcome
We should be able to whitelist new assets in bulk.

Example Group Proposal with multiple assets

{
    "group_policy_address": "zeta1dlszg2s",
    "messages": [
        {
            "@type": "/zetachain.zetacore.crosschain.MsgWhitelistERC20",
            "creator": "zeta1dlszg2sst9r69my4f84l3mj66zxcf3umcgujys30t84srg95dgvs5wguxq",
            "erc20Address": "0xcbB7C0000aB88B473b1f5aFd9ef808440eed33Bf",
            "chainId": "8453",
            "decimals": "8",
            "name": "ZetaChain ZRC20 cbBTC on Base Mainnet",
            "symbol": "CBBTC.BASE",
            "gasLimit": "100000"
        },
        {
            "@type": "/zetachain.zetacore.crosschain.MsgWhitelistERC20",
            "creator": "zeta1dlszg2sst9r69my4f84l3mj66zxcf3umcgujys30t84srg95dgvs5wguxq",
            "erc20Address": "0xcbB7C0000aB88B473b1f5aFd9ef808440eed33Bf",
            "chainId": "1",
            "decimals": "8",
            "name": "ZetaChain ZRC20 cbBTC on Eth Mainnet",
            "symbol": "CBBTC.ETH",
            "gasLimit": "100000"
        }
    ],
    "metadata": "",
    "title": "Whitelist ZetaChain ZRC20 cbBTC on Base mainnet",
    "summary": "Whitelist ZetaChain ZRC20 cbBTC on Base",
    "proposers": [
        "zeta14m8s4x036jkjnfnkw92004zweh5prqzq69n880"
    ]
}