Create config and secret store consolidation spec

[ChristianPavilonis]

Parent: #684

Create a proposal/spec document for consolidating Trusted Server runtime storage around one Config Store alias and one Secret Store alias.

Scope:

• Define the target ts_config_store and ts_secrets runtime namespace model.
• Capture request-signing migration from jwks_store, signing_keys, and api-keys.
• Define Fastly provisioning validation and compatibility behavior.
• Keep the work as a design/spec before implementation.