SPM Prep – Use WordPressComRESTAPIInterfacing in the Objective-C consumers

[mokagio]

Description

The bulk of this PR is going from wordPressComRestApi GET|POST to wordPressComRESTAPI get|post. That is, from the concrete implementation to the one referenced via the new WordPressComRESTAPIInterfacing protocol introduced in #760 .

Other than that, there are two notable changes:

• Added baseURL: URL property to WordPressComRESTAPIInterfacing, 72c7132
• Updated the methods in the protocol to return NSProgress *. This was always the behavior in the original WordPressComRestApi, but because the return value is discardable, I didn't notice it till I run into code that required it, 83d03ab

Testing Details

Green CI 👌

Next up

Basically, cherry-pick the work from #738 but in a neat and reviewable order:

• Move files into Sources/ and Tests/
• Introduce Bundle helper that differentiates between SPM and CocoaPods installations
• Isolate core API objects in dedicated package

See also, #756, #758, #760 which are related to the SPM work but independent from this one.

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