Feature add support for attributes in Project and add AttributeManager

[N0K0]

Hi! Noticed that settings attributes was not possible (or am i potentially blind? :D)

I have not added new tests as of now. Also: I'm a bit confused if there might be some issues with your API, is the attributes endpoint available under orangization ID and not just the shortform?

IE https://snyk.io/api/v1/org/dae[...]fd10c/project/25d[...]dd5b/attribute/remove vs https://snyk.io/api/v1/org/backend-xxx/project/25d[...]dd5b/attribute/remove