Add stage_file_proxy stream wrapper

Not sure if it matters but 🐛 Add a lock around retrieving upstream files Fixed was merged

I tried patch #67 with version 1.1 and private images fail to download from origin server. Since there is no patch for version 2, is there any other approach to make private files work?

Patch #67 on stage_file_proxy 8.x-1.1 worked for me. However, The application that I am working on serves the private files from the overridden path(not system/files). Hence I had to save the patch locally and make below changes:

• The private files directory in my application is configured via settings.php in $settings['file_private_path'], So I updated the FetchManager.php file's code $file_dir = $this->fetchInfoService->getLocalSchemePath($fetchInfo->getScheme()); to $file_dir = \Drupal\Core\Site\Settings::get('file_private_path');. This ensures that stage_file_proxy saves the private files at the location from which they are read/served.
• The functions getUrlBasePath() & getFetchInfo() of SfpPrivateStream.php class has hardcoded the paths system/files & system/files/styles respectively. However, In my case, the files were being served from different paths and hence I updated it to suit my need.

The above changes made the patch #67 work for me.

This is the last ticket needed for the 2.0.3 release fyi.

@smustgrave it may be worth carrying on with 2.0.3 without this one, and adding this in when it's ready.

I did my best to try to reroll this to version 2.1.1, there is quite a bit of code refactoring to 2.0 that made it a challenge. This is working, but probably could use some more tweaking + testing + cleanup. One thing I've notice is that some of the code from 1x. is no longer in 2.x like the webp support code.

Started a new branch for just this feature
Rerolled what I could 🐛 Error retrieving images from styles that use ConvertImageEffect Fixed will have to be tested again as that was using the variables we removed.
Also removed all deprecated functions and interfaces.

Needs some work for upgrade paths but StageFileProxyServiceProvider needs some work as it's breaking the module from being installed and I can't see why.