Improve the separation of URL construction & execution concerns

Expected behavior

Request construction & execution should be separate concerns.

Actual behavior

As discussed in #78, responsibilities for request construction are shared between both ServiceRemoteWordPressComREST & WordPressComRestApi. WordPressComRestApi currently executes requests.

We might benefit from the approach taken by our Woo counterparts.

Steps to reproduce the behavior

N/A

WordPressKit Environment

All devices & versions of WordPressKit, Xcode & iOS.

• WordPressKit Version [e.g. 22] : all versions
• OS: [e.g. iOS 12.1.3 (16D40)] : all versions
• Device: [e.g. iPhone XS] : all devices
• Xcode Version: [e.g. Xcode 10.1 (10B61)] : all versions

Additional Context

N/A