feat: Apple SDK update for version 16.1.0

[ChiragAgg5k]

This PR contains updates to the Apple SDK for version 16.1.0.

[greptile-apps]

Greptile Summary

This PR bumps the Apple SDK to version 16.1.0, updating version headers, adding the x OAuth provider, adding the userType field to the Log model, and exposing a getHeaders() method on Client. All changes are consistent with the existing code patterns, and documentation (CHANGELOG, README) is updated correctly.

Confidence Score: 5/5

This PR is safe to merge — changes are straightforward, well-scoped, and consistent with existing patterns.

All five files contain only expected, mechanical changes for a minor SDK version bump: version string updates, a new enum case, a new model field applied uniformly across all serialization paths, and a simple getter method. No logic errors, missing cases, or security concerns were identified.

No files require special attention.

Important Files Changed

Filename	Overview
Sources/Appwrite/Client.swift	Version headers bumped to 16.1.0/1.9.1; new getHeaders() method added to expose the headers dictionary
Sources/AppwriteEnums/OAuthProvider.swift	New case x = "x" added alphabetically; consistent with existing enum patterns
Sources/AppwriteModels/Log.swift	New non-optional userType: String field added across CodingKeys, property, init, decode, encode, and from(map:); force-cast in from(map:) will crash if key is absent
CHANGELOG.md	16.1.0 entry added with all relevant changes documented
README.md	API version badge and Swift Package Manager minimum version reference updated to 1.9.1 / 16.1.0

_{Reviews (1): Last reviewed commit: "chore: update Apple SDK to 16.1.0" | Re-trigger Greptile}