Update storage and avatars services to include methods to get URLs

[stnguyen90]

Issue: #71

SDK Generator PR: appwrite/sdk-generator#495

[stnguyen90]

@lohanidamodar, you can focus on 9a770bc to see the changes to the storage and avatars services. Hopefully, this gives you an idea of what I was thinking we would provide. We can finalize the name of the methods and docs before merging.

[Shiba-Kar]

Done This

appwrite:
    git:
      url: https://github.com/appwrite/sdk-for-flutter.git
      ref: feat-71-method-generate-file-url

getting this error

('../../../../../AppData/Local/Pub/Cache/git/sdk-for-flutter-9a770bc75904727660acc7b58a1a9344e8ae68dc/lib/src/client.dart').
[        ] Try correcting the name to the name of an existing getter, or defining a getter or field named 'endpoint'.
[        ]         final uri = Uri.parse(client.endpoint + path).replace(queryParameters: params);

We want to get the image Url its a must have feature.

[stnguyen90]

We want to get the image Url its a must have feature.

@Shiba-Kar, this is a pretty stale branch and only a POC. I would suggest building the URL manually instead of trying to use this branch.