Feature request: check for valid `//axis/xyz/@expressed_in` values

## Desired behavior

The `gz sdf --check` command checks for error in an SDFormat file, including checks in many contexts (`//pose/@relative_to`, `//frame/@attached_to`, `//joint/parent`, `//joint/child`, etc) that references to frames match the name of a frame in that scope. I don't see any checks that the `//axis/xyz/@expressed_in` values point to valid frames. We should add a check for this in `gz sdf --check` and wherever else it is appropriate.

## Alternatives considered

## Implementation suggestion

## Additional context