contracts-bedrock: cleanup FeeVault

[tynes]

Description

Updates the FeeVault to follow modern conventions used in the repo
by moving to usage of interfaces rather than implementations. Also
moves the FeeVault into the L2 package as its only really useful
on L2. This is meant to reduce the diff for the Stanard L2 Genesis
by breaking up the refactor into its own small PR.

[semgrep-app]

Semgrep found 1 sol-style-doc-comment finding:

• packages/contracts-bedrock/src/safe/SafeSigners.sol
  • L29-34 - Triage

Javadoc-style comments are not allowed. Use /// style doc comments instead.

_{Ignore this finding from sol-style-doc-comment.}

[tynes]

@smartcontracts I added the fee vaults to the ignore list in the interface script.

They were giving the following error:

       {
         "name": "_withdrawalNetwork",
         "type": "uint8",
-        "internalType": "Types.WithdrawalNetwork"
+        "internalType": "enum Types.WithdrawalNetwork"
       }

This requires normalization, which given you are rewriting this script in Go, I didn't want to duplicate the work. This issue arises even if the WithdrawalNetwork is defined in the FeeVault, since the interface autogen with cast creates a library specific to the interface director when the enum is defined outside of the interface.

I recommend we think a bit from first principles rather than just staying within the confines of cast's behavior, ie we could ask for an update to cast interface or directly generate the interface files with our own tooling