Support metadata encryption

### Is your proposal related to a problem?

The recommended approach for adding a data source via the metadata api is to reference an environment variable instead of providing the connection string (or url) directly in the payload, otherwise the connection details are stored in plain text in the database. However, if using environment variables, the graphql-engine needs to be restarted each time a new data source is added. For our use-case we need to add new data sources frequently and dynamically. Having to restart for each one will not work in this situation.

### Describe the solution you'd like

Add support for encryption of hasura metadata.

### Describe alternatives you've considered

We've not found a suitable alternative yet.

### If the feature is approved, would you be willing to submit a PR?

No.