asynchronously get assets from gee

[dfguerrerom]

As mentioned in this issue, the AssetSelect component experiences significant delays during loading. This occurs because the sepal_ui.scripts.gee.get_assets function recursively calls the Earth Engine API for each folder at every level. The more folders present, the longer the loading time becomes.

To improve performance, we should implement asynchronous calls to the Earth Engine API to fetch assets more efficiently.

this will also help to improve #870

[dfguerrerom]

in #934 I have proposed an asynchronous solution for this problem, however, due to the synchronous naturalness of the earthengine-api there's no way to do fully asynchronous calls in one single thread, that's why there I dispatch threads, which is okaish but not ideal...

I found this nice repository https://github.com/omarryhan/aiogoogle which makes async calls to Google API's, however, earthengine uses a different discovery url, which we cannot use ATM: omarryhan/aiogoogle#147.

That problem can be easily bypassed by manually requesting the discovery document and replacing the GoogleAPI

discovery_document = requests.get("https://earthengine.googleapis.com/$discovery/rest?version=v1").json()
earth_engine_api= GoogleAPI(discovery_document, validate=True)

After that we could just do the requests in the way of earth_engine_api.projects.assets.listAssets(parent=parent)

Last thing to check is the authentication, aiogoogle uses user_creds and client_creds to start the session, and I would like to get them in a transparent way from ee.Initialize(), (if there's a way)