What should be the key_id value?

The Google tokens are now encrypted and saved in Postgres using Fields.Encrypted:

https://github.com/dwyl/app-mvp-phoenix/blob/9e1bf7dd5ac0b073f892ea9b93346f991a817407/lib/app/ctx/session.ex#L5-L12

However we do not save the `key_id` value yet.

I don't fully understand yet how the key value is selected. It seems to be done automatically by Fields, so I need to read the code of Fields to see how it's done and then to see if we need to select the key in the app itself first instead of letting Fields select it automatically.