chore: extract method to check Path is S3Path

[guicamest]

Description of changes:

Considering that in many places a check is done to verify the path (from the nio public API signature) is an S3Path, the changes introduce a method checkPath to avoid repetition of the code, better reusability and consistency (checks were done differently depending on the method).

The implementation is quite similar to the one used in UnixFileSystemProvider which is quite straight forward, replacing the path implementation with S3Path:

    UnixPath checkPath(Path obj) {
        if (obj == null)
            throw new NullPointerException();
        if (!(obj instanceof UnixPath))
            throw new ProviderMismatchException();
        return (UnixPath)obj;
    }

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[markjschreiber]

Thanks for this PR. A few minor changes requested.

[markjschreiber]

Thanks again. Approved and merged!