[Refactor] Common transaction creating apis

[KnowWhoami]

Currently we have different types of apis which focuses on creating a particular types of transactions.
for eg

• spend_from_wallet api which is used to general purpose transaction.
• create_funding_txes -> which creates funding txs for starting coinswap
• create_fidelity -> creates fidelity bond by first creating , broadcasting the fidelity tx and makes further updates.
• create_senders_contract_tx -> creates contract tx which spends utxos generated by funding txes.
  and many more...

But all these api's do same work at some level -> i.e they all contain reductant code for creating a transaction , selecting required utxos etc.
This redundancy can be minimised by following proposed structural design:

fn create_signed_tx(fee:Amount, amount:SendAmount, destinations:Destination, coins_to_spend)-> Result<Transaction, WalletError>{
   // private function
   // A general tx generating api which will create any type of transactions , sign it and return tx instance.
   
   // implementation -> same as `spend_from_wallet` api.
   
   return transaction
}

pub create_fedility{

   // do some work  related to fedility.

   // select required utxos for making fidelity bonds.
   let selected_utxos= self.coin_select(..);

   // create tx instance and sign it.  
   let signed_tx= create_signed_tx(...);

   // do some work related to fidelity.   
}


// .... Similarly we can create other specific transaction api which will be wrappers .

Enhancements:

• Remove reductant code which focuses on creating transaction struct and selecting required utxos.
• Improve code quality and managability.

[KnowWhoami]

I am taking this up.