Provide a way to specify user&password in two different properties

[otbe]

This would enable us to provide user&pw via something like awsspring and also via mechanisms like ECS Secrets. Additonally, we can use the same source on the server side where we have to specify user: password via properties or programmatically.

One could easily provide a backwards compatible way by introducing factcast.grpc.client.user and factcast.grpc.client.password and checking both places.