Add ability to read secrets from file

[toomyem]

Describe the problem to be solved

It would be beneficial if secrets could be read from a file, not just from an environment variable. I'm deploying Grist in docker swarm and would like to have for example OIDC client secret stored as docker secret.

Describe the solution you would like

Maybe, if GRIST_OIDC_IDP_CLIENT_SECRET is not set, then check GRIST_OIDC_IDP_CLIENT_SECRET_FILE and read content of a file pointed by this variable. It is a common approach, see for example Postgres and POSTGRES_PASSWORD_FILE.

We could add a generic solution to AppSettings.ts:read() to look for VAR_FILE if VAR is not found.