Adds v2 `get_private_keys` method for offline / multiaddress output

[smk762]

KDF PR: GLEECBTC/komodo-defi-framework#2542

Example error messages collected so far:

{
    "mmrpc": "2.0",
    "error": "Internal error: Unsupported protocol for ATOM: TENDERMINT(TendermintProtocolInfo { decimals: 6, denom: Denom(\"uatom\"), min_balance_for_ibc_routing: None, account_prefix: \"cosmos\", chain_id: chain::Id(cosmoshub-4), gas_price: None, ibc_channels: {\"osmo\": ChannelId(141), \"iaa\": ChannelId(91)} })",
    "error_path": "offline_keys",
    "error_trace": "offline_keys:331]",
    "error_type": "Internal",
    "error_data": "Unsupported protocol for ATOM: TENDERMINT(TendermintProtocolInfo { decimals: 6, denom: Denom(\"uatom\"), min_balance_for_ibc_routing: None, account_prefix: \"cosmos\", chain_id: chain::Id(cosmoshub-4), gas_price: None, ibc_channels: {\"osmo\": ChannelId(141), \"iaa\": ChannelId(91)} })",
    "id": null
}

{
    "mmrpc": "2.0",
    "error": "Failed to derive keys for BBK: Derivation path not defined for this coin. HD mode requires a valid derivation_path in the coin configuration.",
    "error_path": "offline_keys",
    "error_trace": "offline_keys:334]",
    "error_type": "KeyDerivationFailed",
    "error_data": {
        "ticker": "BBK",
        "error": "Derivation path not defined for this coin. HD mode requires a valid derivation_path in the coin configuration."
    },
    "id": null
}

{
    "mmrpc": "2.0",
    "error": "Coin configuration not found for COVID",
    "error_path": "offline_keys",
    "error_trace": "offline_keys:311]",
    "error_type": "CoinConfigNotFound",
    "error_data": "COVID",
    "id": null
}

{
    "mmrpc": "2.0",
    "error": "HD index range is too large: maximum range is 100 addresses",
    "error_path": "offline_keys",
    "error_trace": "offline_keys:552]",
    "error_type": "HdRangeTooLarge",
    "id": null
}

{
    "mmrpc": "2.0",
    "error": "HD index range is invalid: start_index 40 must be less than or equal to end_index 0",
    "error_path": "offline_keys",
    "error_trace": "offline_keys:548]",
    "error_type": "InvalidHdRange",
    "error_data": {
        "start_index": 40,
        "end_index": 0
    },
    "id": null
}

{
    "mmrpc": "2.0",
    "error": "Error parsing request: invalid type: floating point `4.294967299566574e27`, expected u32",
    "error_path": "dispatcher",
    "error_trace": "dispatcher:126]",
    "error_type": "InvalidRequest",
    "error_data": "invalid type: floating point `4.294967299566574e27`, expected u32",
    "id": null
}