feat: server overrides

[greghuels]

Fixes: FF-3995

Motivation and Context

Allows users to override server-side flags by providing new parseOverrides and withOverrides functions to the EppoClient.

• parseOverrides - Allows users to parse the x-eppo-overrides header sent by web requests. Eppo's Chrome extension will intercept and append this header to all https endpoints for the target site.
• withOverrides - Creates an EppoClient instance that has overrides applied so that it doesn't affect the behavior of the original EppoClient singleton on a shared Node web server.

Typical usage will be something like this:

export const eppoOverridesStorage = new AsyncLocalStorage<Record<FlagKey, Variation>>();

@Injectable()
export class EppoOverridesMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    getInstance()
      .parseOverrides(req.headers['X-EPPO-OVERRIDES'] as string | undefined)
      .then((overrides) => eppoOverridesStorage.run(overrides, next))
      .catch(next);
  }
}

export const getEppoInstance = () => {
  const overrides = eppoOverridesStorage.getStore();
  return EppoSDK.getInstance().withOverrides(overrides);
};

How has this been tested?

• Manual QA
• TODO: Additional unit testing

[greghuels]

The final version of this will hit a new endpoint that validates the "browser extension" key