fix(api): Support legacyFallback for BUILD_SERVICE

[ajordens]

When service.fiat.legacyFallback is true, this will implicitly allow
access to all build services in the event that fiat is unavailable.

[ajordens]

@dibyom We are currently going through an exercise to understand the situation if (and when) fiat were to become unavailable.

Since our adoption, we've always been supporting a legacyFallback model wherein we effectively fall back to the state that existed prior to fiat (account restrictions only).

This may need revisiting at some point but it's a bare minimum that we'd like to see supported for now.

[ajordens]

Also thinking that there may be a model where source of truth systems (ie. jenkins, clouddriver) could supply their own fallbacks (or an extension point allowing a user to supply their own fallback logic).

[dibyom]

LGTM! The legacyFallback makes sense for clouddriver accounts. Will this allow access to all build services to all users if Fiat is not available?

[ajordens]

@dibyom If enabled, it would.

We've bounced around the idea for an optional extension point (likely in igor) that could potentially blacklist certain build services even for fallbacks.