use `sdk.Int` for TxData's chain-id and amount

[fedekunze]

Now that the SDK version supports the max ERC20 value (cosmos/cosmos-sdk#9511), we should update the proto message fields from TxData to use sdk.Int instead of plain bytes.

We should also update any relevant validation functions

Scope:

• Update proto message
• Update validation functions, constructors, etc
• Remove (Un)MarshalBigInt funcs from types/