Fix error: create account missing signatures field

[karczuRF]

So far in TU we used accounts.create_free_test_coins to create an account. The reason why we do it in this way was quite a long time ago discussed in this issue: #80 . The problem is that this function doesn't work anymore- I got the error Transaction API error: Network interface error: Indexer client error: Request failed: code: -32602 message: missing field 'signatures',.
If you run Tari Dan Wallet and send request {"method":"accounts.create_free_test_coins","jsonrpc":"2.0","id":5,"params":{"account":{"Name":"test"},"amount":200000,"max_fee":1000,"key_id":null}} the account is created.

Last update: investigate if transaction model is up-to-date and submitting signatures instead of signature