[Data Liberation] Create WP entity stream API for use by exporters

[brandonpayton]

This is a tracking issue.

For data liberation, we want an API for streaming WP entities from a site. Then we can export WP entities to multiple targets without having to solve the same entity traversal problems in each exporter. Some example exporters are:

• WXR
• WXR + assets package
• Static HTML files
• Markdown

The idea is to present the API as an entity iterator with a constructor that supports reentrancy.

Let's focus on a single, uninterrupted traversal for the first version of this API. I think we can implement reentrancy in a straightforward manner once the traversal flow is established.

[brandonpayton]

The next step is to create a draft PR for this API. If there end up being a lot of questions, I'll start a separate GH discussion where we can make a mess and figure things out.