Ensure OAuth credentials are fresh before using them in graph execution

[Pwuts]

The ExecutionManager fetches the needed credentials for a graph excution from Supabase, but it should also refresh them if needed using the appropriate OAuthHandler.

[Pwuts]

This is made more complex by a few factors:

• When a refresh token is used, the corresponding access token becomes invalid. So if one part of the system refreshes the token, this needs to be propagated to all other parts of the system.
• Tokens refresh can (in principle) be initiated by both the executor and the rest_api server.

The only way I see to properly deal with this is to have centralized credentials management, and JIT retrieval of credentials for use.