[Feature Request] OIDC Client Secret as ENV

Description Currently the PAPERLESS_SOCIALACCOUNT_PROVIDERS is the documented way to configure any and all OIDC or OAuth clients. This lumps all of the secure fields together into a single environment variable which is awkward when managing multiple providers or doing anything automated. In my setup, Client Ids and Secrets are generated by an operator and stored as a secret independent of paperless-ngx. I do not want to copy these values from one secret into a new secret or env, as that breaks the workflow and can potentially get out of sync. I can mount these variables in the paperless container like below, but currently this does not achieve anything. it would be useful to be able to specify the secret value as a single env variable, like I have done above, and have paperless use the Client ID and Secret from the variables. It doesn't have to be as via the existing account_providers variable, but some way to avoid copying the secret value out of one variable and into…