fix: include userIdentifier in Session Replay Identify payload (#220)

## Summary
- Mobile `Identify` Session Replay events rendered an empty description
in the session player. The player resolves a description from `email ||
name || userIdentifier`, but the SDK only emitted `key` and
`canonicalKey`.
- Emit `userIdentifier` (friendly name when set, otherwise the canonical
key) in the Identify payload so mobile identifies show a description,
matching the web `Identify` payload shape (`{ userIdentifier,
...userObject }`) without any frontend changes.

## Notes
- `email`/`name` are intentionally not plumbed: they aren't available at
the identify hook call site (only `contextKeys` + `canonicalKey` are
forwarded), and `userIdentifier` is sufficient to fix the
empty-description bug.

## Test plan
- [ ] Build the package
- [ ] Trigger an identify and confirm the Session Replay Identify event
renders a non-empty description in the session player

Made with [Cursor](https://cursor.com)

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: abelonogov-ld

Sources/LaunchDarklySessionReplay/Exporter/IdentifyItemPayload.swift

@@ -51,8 +51,10 @@ extension IdentifyItemPayload {
         if let contextFriendlyNameUnwrapped = options.contextFriendlyName, contextFriendlyNameUnwrapped.isNotEmpty {
             contextFriendlyName = contextFriendlyNameUnwrapped
         }
-        attributes["key"] = contextFriendlyName ?? canonicalKey
+        let displayKey = contextFriendlyName ?? canonicalKey
+        attributes["key"] = displayKey
         attributes["canonicalKey"] = canonicalKey
+        attributes["userIdentifier"] = displayKey
 
         return attributes
     }