Provide method to return an asset file as a stream

Problem/Motivation

Some file/asset formats (such as watermarked) should be accessed as streams, not as URLs, and indeed cannot be retrieved by means of the AssetLink API.

We need to provide a means for retrieving such files dynamically, as a stream, so that an integration builder can decide how to store or migrate or display the file.

Proposed resolution

Add new AssetFile Endpoint to the orange-dam-php library..

Create method in OrangeDamApi that will return a stream, and minimal necessary metadata (mimetype, size). Perhaps wrap this in a custom object with an interface for ease of use.

We will also see if we can begin to sketch out an abstract general purpose "repeat request" function that the other data retrieving functions can also (later) use, so that we have less code repetition.

API changes

Adds new method OrangeDamApi::getAssetFile()

Data model changes

none