feat: add call gemini for v2

[HYACCCINT]

No description provided.

[jhuleatt]

Before calling Gemini, let's look for both appcheck and auth, but gate each with its own boolean parameter (https://firebase.google.com/docs/functions/config-env?gen=2nd#parameter_types) so devs can choose whether they want auth/app check or not.

Something like:

// ...imports

const appCheckRequired = defineBoolean("APP_CHECK_REQUIRED");
const authRequired = defineBoolean("AUTH_REQUIRED");

// ...other setup

exports.callGemini = onCall(
  {
    enforceAppCheck: appCheckRequired, // Reject requests with missing or invalid App Check tokens.
  },
  (request) => {
    if (authRequired.val() && !request.auth) {
      throw new HttpsError(
        "failed-precondition",
        "The function must be called while authenticated.",
      );
    }

    // ...call Gemini
  },
);

[jhuleatt]

please remove before merging

[jhuleatt]

is node-fetch needed for this sample?