Use Environment Variables for Google OAuth Client Credentials

[nelsonic]

At present, the instructions in the README.md suggest putting the google_client_id in the config.exs file:

Could we store both the GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET in the environment (or in an .env file) and load it at run-time instead of hard-coding them in the config.exs file?

[nelsonic]

• https://github.com/dwyl/fields/blob/90779dff6713c49a335e31b218a0586f3a2faa89/config/config.exs#L33-L42
• https://github.com/dwyl/phoenix-ecto-encryption-example/blob/09c0a24972eee3e1262e1129a18cc60e3ea53098/config/config.exs#L43-L47

[nelsonic]

PR #11 adds detailed instructions for environment variables for Google API keys. ✅