comment: add TODO comment for ApprovalRequest type to use ApprovalType (

#4209)

## Explanation

We are receiving a typescript error in metamask-extension when comparing
ApprovalRequest.type value to ApprovalType values (example:
MetaMask/metamask-extension#24182)
<img width="1045" alt="Screenshot 2024-04-24 at 3 37 16 PM"
src="https://github.com/MetaMask/core/assets/20778143/5bec8d91-1581-4cab-9faa-bed441784d69">
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### `@metamask/approval-controller`

- **comment**: add TODO comment for ApprovalRequest type to use
ApprovalType


## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

---------

Co-authored-by: Jongsun Suh <jongsun.suh@icloud.com>

packages/approval-controller/src/ApprovalController.ts

@@ -90,6 +90,8 @@ export type ApprovalRequest<RequestData extends ApprovalRequestData> = {
 
   /**
    * The type of the approval request.
+   * Unfortunately, not all values will match the `ApprovalType` enum, so we are using `string` here.
+   * TODO: Replace `string` with `ApprovalType` when all `type` values used by the clients can be encompassed by the `ApprovalType` enum.
    */
   type: string;