Update getNep21Balance with the usage of view method get_bridge_token_account_id of a bridge factory

[alexauroradev]

Currently, in getNep21Balance function we use the following code:

const nep21Address =
    transfer.sourceToken.replace('0x', '').toLowerCase() +
    '.' +
    process.env.nearTokenFactoryAccount

Though at the moment this is correct, a connector might change the way how it maps addresses of the tokens on both sides. For the consistency, we should use get_bridge_token_account_id view method of the bridge factory

[paouvrard]

The frontend could also use a function that goes the other way: bridged_nep141_address -> original_erc20_address so that users can search for a token with the nep141 address when they are transferring from NEAR.

Are these bridged addresses really expected to change ? Similar to token decimals, I think its also nice to be able to know the information without querying everything on chain.