Load content of .env file before merging env

[suttod]

Addresses: authzed/spicedb#1082

[vroldanbet]

Thanks for the contribution!

Viper has native support for configuration files, so I don't believe we need to use another third-party library for this. See:

https://github.com/carolynvs/stingoftheviper/blob/main/main.go#L73-L88
https://maneeshaindrachapa.medium.com/go-with-env-files-using-viper-1eb3d1d1d221

I also couldn't find much about conventions you refer to, as they seem to be different depending on where you look at:

https://gist.github.com/jgarber/20f56d92e549dbde7152
https://terraspace.cloud/docs/config/dotenv/

I'd suggest we start with spicedb.env and then we can follow up if the community seems unanimous on the opinions.

We should also add some tests to make sure precedence is guaranteed.