Revert "Remove old API endpoint" to support BTC sends.

This reverts commit 535fb4db261371c7ad31a0f2f88c1cda324747ce.
Add fee as a required field and pass it to build_transaction. Use old API for btc sends.
Merge branch 'refactor-send-transaction-API' of github.com:Nevtep/omniwallet into refactor-send-transaction-API