Limiit permitted instructions for fee instructions

[sdbondi]

Problem

Currently, a user may submit any fee instructions. This allows them to perform any transaction without being charged for rejected transactions, which results in validators performing non-paying work.

Proposal 1

Limit instructions permitted for fee instructions.

These are the scenarios that should be supported:
a. call pay_fee and pay_fee_confidential on existing account
b. claim burn and use the claimed funds to pay the tx fee
c. claim validator fees and use this to pay the tx fee