gitlab & bitbucket integration

[mike0sv]

Right now we can use github URLs to reference different objects, like https://github.com/org/repo/path
We need to add the same functionality for GL and BB. The easiest way is probably to implement a subset of fsspec AFS and new corresponding UriResolver the same way as for github

[shcheklein]

can we please add a bit more context (description) here + labels? (it's good for open source in general, good for communication and knowledge sharing :) );