Caching of anonymous (guest) credentials

[elpablos]

Why?

• the anonymous endpoint is repeatedly calling one user profile for build in the ID Token
• we can see the load in the Mongo database (a lot of specific reads - thousands per sec)

DoD

• build some in-app caching
• together with Guest (anonymous) credentials provider #235 it could be set up in config on the provider level
• even a 30-second dummy cache could help alot!

Connected issues: #235