Meta: Tx Fee & DEX integration

[xlc]

Depends on #58

First stage:

• Use ACA as tx fee unit.
• Use ACA to pay tx fee.
• For transfer, fallback to transferred currency to pay fee using DEX for conversion.

Second stage:

• New Signed Extension with a byte to indicate which currency to pay for fees and use DEX for conversion.

Final stage:

• Use AUSD as tx fee unit.
  • Still use ACA for payment.