Replace self.caller with self.signer in fee_public

[raychu86]

Motivation

This PR updates the use of self.caller in credits.aleo/fee_public with self.signer. The intention is to enforce that fees must be paid by the signer for safety and clarity.

Programs that import credits.aleo are already unable to call fee_public in an internal function because of this enforcement - https://github.com/AleoHQ/snarkVM/blob/eefd27d8637c78925f25bdb5878e67a9e2addeeb/synthesizer/process/src/trace/mod.rs#L159

This means that self.caller and self.signer must already be equivalent in fee executions, however the change is to make the intention for credits.aleo/fee_public logic extra clear.

Test Plan

A test has been added to check that calls to fee_public and fee_private from external programs are invalid.

Notes

A new genesis block must be generated because keys for fee_public have been updated.