Add a typespec linter to detect use of ApiKey (as an insecure auth mechanism) and warn to discourage use in greenfield services

[SFI - Safe Secret Standards](https://eng.ms/docs/products/onecert-certificates-key-vault-and-dsms/key-vault-dsms/certandsecretmngmt/get-started) recommends we discourage the use of ApiKey, connection string, etc. for authentication and instead promote the more secure EntraID/ManagedIdentity as the primary auth mechanisms for services.

A linter to detect and warn for such usage would help service teams.

An example of this is in AppConfig (which is a brownfield service, which needs to keep that auth mechanism for backwards compat):
https://github.com/Azure/azure-rest-api-specs/blob/84f223e8bea800a1a0b503b8c28675abf91f92a2/specification/appconfiguration/AppConfiguration/main.tsp#L14-L22

cc @allenjzhang, @markcowl, @scottaddie

Start by reviewing the linked AppConfiguration/main.tsp example, especially lines 14–22, and the Safe Secret Standards guidance. Define how the linter should identify ApiKey and related insecure authentication usage, how it should warn for greenfield services, and how brownfield compatibility cases should be handled; done means the behavior is specified and validated against the example.

Written by the indexing model from the issue text.