[WIP] Added support for resolving vault secrets on-demand basis

[VShingala]

Problem Statement

Currently, vault secrets must be pre-loaded as a VariableScope with all values passed upfront via options.vaultSecrets. This requires the consumer (e.g., Postman App, Newman) to:

Know all vault secrets that might be needed before the run starts

Fetch all secrets synchronously before runtime initialization

Handle secret refresh/expiry outside of the runtime flow

Solution

Introduce an on-demand resolution mechanism where:

Runtime detects which vault variables are referenced (e.g., {{vault:secret_key}})

Calls an async resolver function provided by the consumer to fetch those specific secrets

Resolves secrets at the point of need (request resolution, script execution)

Maintains existing domain-based filtering behavior