feat(file-loader): Add recursive loading from sub-directories

[EshaanAgg]

What type of PR is this?
Adds support for recursive file loading.

Which issue(s) this PR fixes:
Fixes #4689

Release Notes: Yes

new features: |
  Added support for recursive subdirectories in the FileProvider API

[EshaanAgg]

@shawnh2
I have added the tests for some basic test cases in the case of nested directories in the new test suite. Since fsnotify does not support watching recursive directories, I listed all the sub-directories of the provided directories in paths, and then added a watcher on all of them.

I am currently a bit unsure about two things:

• How should we be handling the various change events in this region of code:
  

  gateway/internal/provider/file/file.go

  Lines 151 to 159 in d611d33

  	switch event.Op {
  	case fsnotify.Create, fsnotify.Write, fsnotify.Remove:
  	// Since we do not watch any events in the subdirectories, any events involving files
  	// modifications in current directory will trigger the event handling.
  	goto handle
  	default:
  	// do nothing
  	continue
  	}

  
  Leaving it as it allows the current test suite to pass, so I am not certain how we should handle it.
• Should we add tests for cases where the user creates, deletes, or renames a complete directory in one of the watched directories, or is that unnecessary? Do we intend to support that functionality right now?

Sorry for the trouble associated with reviewing a new PR. And thanks! :)