[PSR-7] Add getMetadata() to StreamInterface

- Adds a `getMetadata()` method to the StreamInterface, as suggested by
  @mtdowling, and as originally defined at
  https://github.com/guzzle/streams/blob/3.0/src/StreamInterface.php#L142-L157
Merge pull request #332 from weierophinney/feature/stream-get-metadata

[PSR-7] Add getMetadata() to StreamInterface