Fee validation refactor

[grarco]

Describe your changes

Refactors wrapper_fee_check into three different functions, one for each consensus state (CheckTx, PrepareProposal and ProcessProposal). Extract the fee unshield processing to a separate function. Also changes some methods of Shell to just be functions since we don't really need a reference to the shell.

Indicate on which release or other PRs this topic is based on

v0.29.0

Checklist before merging to draft

• [x] I have added a changelog
• [x] Git history is in acceptable state