Environment variables for Google Play credentials

[lrockreal]

Closes #213

Adds environment variables for Google Play credentials: APKEEP_GOOGLE_EMAIL, APKEEP_GOOGLE_OAUTH_TOKEN and APKEEP_GOOGLE_AAS_TOKEN.

Implementation details:

• Credential loading priority goes CLI args -> env vars -> config file -> prompt
• AAS token takes precedence over OAuth token if both are provided
• OAuth env var requires explicit provider selection
• Config file now always checked for email/AAS token regardless of selected provider

[Hainish]

I'm curious: why move the config file check up to the broader scope? It is only needed when download_source is DownloadSource::GooglePlay and we are not listing versions (since Google Play doesn't list versions). Moving it to this broader scope seems to add unnecessary logic and filesystem reads when they are not needed for other download sources.