Allow for different ways of providing consumer_key config

[J4bbi]

Description

There is currently a hard-coded reference to the optional app_key parameter of the Flask-OAuthlib library. Resulting
in a KeyError being raised if it is not present.

invenio-oauthclient/invenio_oauthclient/handlers/utils.py

Line 86 in 4aaa9c5

remote_app_config = current_app.config[remote_app["params"]["app_key"]]

Using the app_key is a way to lazy load the consumer_key and consumer_secret, but it is possible to
provide them directly. It will have to be either, otherwise the remote app won't load.

This PR also removes a duplication of defining the remote_apps variable and fixes some documentation.

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