Azure authentication is token based - the client obtains an access token
with their credentials, and uses it as a bearer token to authorize
requests to Azure services.
Define a common API for all credential types. The API will consist of a
single `get_access_token()` function that will be returning a new or a
cached access token for some resource URI (defines token scope).
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>
