Extend RemotePostService with new create and patch endpoints

[kean]

Description

This PR adds PostServiceRemoteExtended with new APIs for creating and patching posts:

public protocol PostServiceRemoteExtended: PostServiceRemote {
     func createPost(with parameters: RemotePostCreateParameters) async throws -> RemotePost
     func patchPost(withID postID: Int, parameters: RemotePostUpdateParameters) async throws -> RemotePost
 }

In the previous implementation, you had to use the response object (RemotePost) as an input parameter for updating the posts, which was less than ideal because of the mismatch between the set of available parameters and their format:

- (void)updatePost:(RemotePost *)post success:(void (^)(RemotePost *post))success failure:(void (^)(NSError *error))failure;

The new parameter formats also support creating diffs between two RemotePostCreateParameters and applying a diff to RemotePostCreateParameters. Both of these features are used by the updated PostRepository in WP-iOS.

Testing Details

See wordpress-mobile/WordPress-iOS#22807

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• Please check here if your pull request includes additional test coverage.
• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[dangermattic]

	1 Warning
⚠️	This PR is larger than 300 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

Generated by 🚫 Danger

[momo-ozawa]

Tested via wordpress-mobile/WordPress-iOS#22807

[crazytonyli]

👋 Hi @kean, would you mind adding some unit tests?

[kean]

Hey, we have a suite of integration tests in the following PR. It has complete coverage for this change, including the underlyingError addition, diff, and both the new methods.

[crazytonyli]

They are in a separate repository though. It'd be nice to have complete coverage on these changes in this repo here too.