Enhance `createrune` to support restrictions for the RPC's `readonly` category as well

When creating a rune, restrictions will be applied based on the category of the RPC method it grants access to.

For example:

- If an RPC method belongs to the "readonly" category, a rune with category restrictions will be permitted to access it.

- However, if an RPC method does not fall under the "readonly" category, a rune with such restrictions will not be allowed to access it.