[FR]: Add inMemory support for Auth module

### Description

**1. Use Case:**
As a developer, I need to configure the Firebase Auth module to utilize in-memory storage exclusively within the UI Tests target. Since UI Tests cannot access the Keychain, this adjustment is critical for validating UI state changes triggered by Firestore snapshot listeners. Specifically, I require the ability to programmatically authenticate users during UI tests to bypass security rules and verify correct UI behavior when interacting with Firestore data protected by authenticated user permissions.

**2. Current Firebase SDK Behavior:**
The Firebase Auth SDK defaults to secure Keychain storage for sensitive user data (tokens, credentials) on iOS/macOS. While this ensures production-grade security, it renders Auth unusable in UI Testing environments due to Keychain access restrictions. This limitation blocks testing of auth-dependent flows, such as Firestore write operations guarded by security rules.

**3. Proposed Enhancement for Firebase SDK:**
Introduce an **explicit in-memory storage** mode for Firebase Auth, configurable at runtime. This mode would:

**Disable Keychain Persistence:** Prevent any Keychain read/write operations when activated.
**Retain Auth State In-Memory:** Maintain user credentials, tokens, and auth state within the active app session.
**Reset State on App Termination:** Clear all auth data upon app exit to mimic ephemeral test environments.
Implementation Recommendations:

**Add** a `storageType` Property to `AuthSettings`:

```
public enum AuthStorageType {
case keychain // Default production behavior (persists to Keychain)
case inMemory // Ephemeral storage for tests (no Keychain access)
}

let auth = Auth.auth()
auth.settings.storageType = .inMemory // Set before auth APIs are used
```

**Documentation & Safety Measures:**
Clearly label `.inMemory` as unsuitable for production in API docs.
Throw a runtime warning/assertion if .inMemory is used outside of debug/UI test builds.

4. **Benefits:**

**Unblock UI Testing:** Enables end-to-end validation of auth-gated workflows (e.g., Firestore writes) without Keychain.
**Explicit Control:** Developers opt into in-memory behavior only where needed, preserving secure defaults.
**Platform Consistency:** Aligns with existing patterns like Firestore’s settings.isPersistenceEnabled.

```
func testAuthProtectedFlow() {
// Configure Auth for in-memory (no Keychain)
let app = XCUIApplication()
app.launch()

// Programmatically sign in (no UI interaction)
Auth.auth().signIn(withEmail: "test@example.com", password: "password")

// Perform Firestore write & validate UI update
let db = Firestore.firestore()
db.collection("protectedData").document("testDoc").setData(["value": 42])
XCTAssert(app.staticTexts["Data Updated"].exists)
}
```

This approach balances security with testing flexibility, empowering developers to validate auth-integrated experiences fully.

### API Proposal

_No response_

### Firebase Product(s)

Authentication