Adds AbstractApiProvider class

[JasonTheAdams]

Resolves #68

This adds an AbstractApiProvider class, which provides a convenient way for API urls to be constructed, cleaning up a bunch of duplicate path constructing code.

I went with an abstract baseUrl method instead of using a constant that gets overridden so that it's clearer to extending classes and doesn't fail at runtime.

[github-actions]

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Co-authored-by: JasonTheAdams <jason_the_adams@git.wordpress.org>
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[felixarntz]

@JasonTheAdams Two tiny but important problems here that need to be addressed before merge, but otherwise this looks good to go, so I'll preemptively approve.

Please feel free to merge as soon as you have addressed these. 🚢