fix(util-body-length-node): restrict fs calls to fs.ReadStream objects

[kuhe]

Issue #, if available:
aws/aws-sdk-js-v3#7408

Description of changes:

In the Node.js body length calculator, only call lstat and fstat if the input body is an actual instance of fs.ReadStream, and not merely possessing a .path or .fd property.

This would break an edge case where a user was expecting a body length calculation on a duck-typed FileReadStream that didn't extend the fs.ReadStream prototype, but that should be rarer than an accidental usage of the path property.

In such cases, the content length should be given by the caller in e.g. the ContentLength property, if available on the operation in question, or changed to extend the ReadStream prototype.