Missing authentication flow update/get

[thalescosta]

Is your feature request related to a problem? Please describe.
Keycloak API has some methods to retrieve or update a given Authentication Flow and this is not implemented in the gocloak

Describe the solution you'd like
Implement these methods to have a similar behavior to other resources.

Describe alternatives you've considered
If someone needs to change a given authentication flow, it needs to retrieve all flows, and delete/create it again.

Additional context
Add any other context or screenshots about the feature request here.