Allow database config to be set via env var

Similar to how database connection string can be set with `from_env: HASURA_GRAPHQL_DATABASE_URL` in `databases.yaml`, we should be able to set

```yaml
max_connections:
from_env: HASURA_GRAPHQL_PG_CONNECTIONS
```

Start with the database configuration described in databases.yaml and trace how the existing from_env setting for HASURA_GRAPHQL_DATABASE_URL is handled. Extend the same configuration path for max_connections and HASURA_GRAPHQL_PG_CONNECTIONS; done means the YAML form is accepted and sets the PostgreSQL connection limit from the environment variable.

Written by the indexing model from the issue text.