Add functionality to accept a JSON template string when initializing a RC server template

[trekforever]

Discussion

During testing, we commonly stored a template as JSON. It was then inconvenient to parse that into an object of a specific type (TemplateData) just to accommodate the type expected by the option.

This change updates initServerTemplate to also accept a JSON string, in addition to the existing ServerTemplateData.

Testing

Unit tested via npm test

API Changes

The InitServerTemplateOptions.template now accepts a union type ServerTemplateData|string instead of just ServerTemplateData

[erikeldridge]

Looking at the other errors we throw, we don't include the raw error (e) in the message. We should check w Lahiru if this is an intentional pattern, and if so, follow it here.

Nit: If we do include the error, it might be more readable to differentiate it from the other error message, something like "... Raw error: ${e}".

[trekforever]

We don't include it for most errors, but we do have it for the createTemplateFromJSON:
https://github.com/firebase/firebase-admin-node/blob/master/src/remote-config/remote-config.ts#L163-L166
(most of this logic is consistent with that method). But will check with Lahiru when I request his review on this

[erikeldridge]

Ah. I overlooked that. Makes sense. Shipit 🚢

[lahirumaramba]

Thank you! LGTM!