Feature request: configure total max connections via environment variable

### Is your proposal related to a problem?

When using the same hasura metadata in multiple environments, it may be desirable to configure different `total_max_connections`,
as the different environments (eg. prod/dev/staging) may be connected to different databases with different characteristics.

### Describe the solution you'd like

Allow setting `total_max_connections` via environment variables.

### Describe alternatives you've considered

The alternative is editing metadata manually before deploying, or using the API to modify this parameter after deploying metadata.

Search the GraphQL engine codebase for `total_max_connections` and the metadata handling for that setting, then trace how environment variables are read and applied. Verify the setting can vary by environment without editing metadata manually or calling the API after deployment, and add coverage for the environment-variable path if the relevant tests are found.

Written by the indexing model from the issue text.