feat: accept Expression for v2 RTDB instance option

[IzaakGough]

Summary

Allows the v2 Realtime Database trigger instance option to be a deploy-time param (Expression<string>), not only a plain string, so functions can target different RTDB instances per environment without TypeScript rejecting valid code.

Fixes #1613

Problem

#1613 reports that ReferenceOptions.instance was typed as string only. Using defineString('REALTIME_INSTANCE') (or similar) for instance failed at compile time, even though multi-environment setups need different instance IDs (e.g. staging vs production).

Solution

• Widen ReferenceOptions.instance to string | Expression<string>.
• In getOpts, when instance is an Expression, resolve it with .value() so the runtime/trigger options get the resolved string (same pattern as other param-backed options elsewhere in the SDK).

Testing

• Added a unit test that defines a string param backed by process.env, passes it as instance, and asserts getOpts includes the resolved instance string.

[gemini-code-assist]

Code Review

This pull request enhances Realtime Database (RTDB) ReferenceOptions to allow Expression<string> for the instance property, enabling dynamic resolution of instance names. The getOpts utility function has been updated to process these expressions, and a new test case validates this functionality. The review suggests improving the new test case by replacing magic strings with named constants for better readability and maintainability.