Add Google Auth support

[emilejimini]

Hello!
I would like to add support for Google auth Single Sign On.

Currently it is possible to add the required environment variables to the api through api.extraEnv but it does not allow to get the env var from a secret.

I would like to be able to configure the an env var for the client id and the client secret in the same way than it is done for smtp: either by a value in values.yaml or by a data field in global.existingSecret.

If it is not considered a major feature, another option would be to create an extraEnvFrom field (in the same way as extraEnv) for each microservice which would allow to create extra env var from configmaps or secrets.

I cannot yet push to a branch to open a PR (error 403): how is it possible to contribute?
Thanks :)