Add patch version to SDFormat spec to account for elements added between minor versions

## Desired behavior

We currently define the specification version in an SDFormat file with a `MAJOR.MINOR` version. The minor version can be updated with new major versions of `libsdformat`, but we periodically add new features to a released version of the specification without increasing the version number. It would be nice to increment the patch version of the specification when new features are added. Then if a file wants to use a feature that is not available in all version of `X.Y`, it can specify `X.Y.Z`. The parser can give a warning or fail if it is passed a file that with an unsupported patch release version.

## Alternatives considered

## Implementation suggestion

## Additional context