feat: bigint support

[andreabadesso]

Motivation

All numbers in rpc methods that can be big numbers should come as strings in the RPC requests, but we should convert them to BigInt before sending to the rpc-lib clients.

Acceptance Criteria

• htr_sendNanoContractTx should accept strings as nano contract action amounts, parse and return BigInt objects to the clients.
• htr_createToken should accept a string with the number as the amount and return a BigInt object to the client.
• htr_sendTransaction should throw a specific error if an error occurs in the prepareTx phase so the clients know whether to show the retry button or not.
• We should have tests to cover all those scenarios.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged
• Make sure either the unit tests and/or the QA tests are capable of testing the new features
• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.