For https://github.com/sourcegraph/sourcegraph/pull/62934 we probably want a local dev mode default that is ergonomic and offline for Enterprise Portal, i.e. doesn't need to connect to SAMS-dev, for everyone who works on Cody Gateway, which will now talk to Enterprise Portal instead. This change adds a special SAMS_API_URL=offline mode that allows all tokens locally, with various safeguards against this being used in any live deployment environment.

Test plan

[enterprise-...l] DEBUG samsm2m.OfflineTokenInstrospector samsm2m/offline.go:28 IntrospectToken request {"scopes": ["enterprise_portal::codyaccess::read", "enterprise_portal::subscription::read"]}