`Insecure TLS Allow List` should support environment variable

### Is your proposal related to a problem?

`Insecure TLS Allow List` only support domain name, however, we do have paired webhook endpoint across different environments. For example, Hasura-DEV -> Webhook-DEV, Hasura-QA --> Webhook-QA, Hasura-PROD --> Webhook-PROD.
The `Insecure TLS Allow List` in managed by metadata, it should use same metadata to deploy on DET, QA and PROD.
It's not make sense to config all domain name(Webhook-DEV, Webhook-QA, Webhook-PROD) together with each Hasura environment.

### Describe the solution you'd like

We'd better support environment variable configuration, so that we could define different endpoint domain name within related Hasura Engine.

Start by locating the configuration entry point for the Insecure TLS Allow List and how its metadata is deployed across Hasura environments. The change is complete when the allow list can resolve an environment-specific endpoint domain while retaining metadata-based deployment.

Written by the indexing model from the issue text.