This class handles the authentication to an openEO back-end that supports "basic" as login type. The class handles the retrieval
of an access token by sending the encoded token consisting of user name and the password via HTTP header 'Authorization'.
The authentication will be done once via login() or multiple times when the lease time runs out. This class
is created and registered in the OpenEOClient(). After the login the user_id and the access_token are obtained and
used as "bearer token" for the password restricted web services.
an object of type R6Class() representing basic authentication
The class inherits all fields and function from IAuth()
$new(endpoint,user,password)the constructor with the login endpoint and the credentials
endpointthe basic authentication endpoint as absolute URL
userthe user name
passwordthe user password