Consolidate `IdItem` and `ISearchable` into `IResourceBase`

All of our main resources should implement the `id` property and the `searchableText` method, so there isn't much reason to keep them separate from `IResourceBase`:
- https://github.com/confluentinc/vscode/blob/45e7fef722a128f31c3560d6b1401a26bde20fc2/src/models/main.ts#L4-L7
- https://github.com/confluentinc/vscode/blob/45e7fef722a128f31c3560d6b1401a26bde20fc2/src/models/resource.ts#L118-L124
- https://github.com/confluentinc/vscode/blob/45e7fef722a128f31c3560d6b1401a26bde20fc2/src/models/resource.ts#L34-L39

See current uses of `IResourceBase & IdItem & ISearchable` [here](https://github.com/search?q=repo%3Aconfluentinc%2Fvscode%20%22IResourceBase%20%26%20IdItem%20%26%20ISearchable%22&type=code)