Implement `AnyNode`/`AnyNodeRef` for `FStringFormatSpec` (#9836)
## Summary
This PR adds the `AnyNode` and `AnyNodeRef` implementation for
`FStringFormatSpec` node which will be required in the f-string
formatting.
The main usage for this is so that we can pass in the node directly to
`suppressed_node` in case debug expression is used to format is as
verbatim text.
