Provide an example for using token-exchange grant type with Keycloak and SonataFlow

[ricardozanini]

The Token Exchange RFC might help users who need our workflows to act on behalf of resource owners. For example, Alice wants to call a workflow to access the Pet Store. The data retrieved by the Pet Store must be related and owned by Alice.

Alice can send a request to the workflow containing her access ID to the Pet Store but not the authentication secrets (e.g., username and password). The workflow will use its service token to exchange for a valid one representing Alice in the Pet Store before calling any REST functions to the resource service.

Resources:

• https://www.keycloak.org/docs/latest/securing_apps/index.html#_token-exchange
• https://datatracker.ietf.org/doc/html/rfc8693
• Token-Exchange Use-cases keycloak/keycloak#26502

[ricardozanini]

Done in the draft PR.