Full Site Editing: Simplify how FSE blocks fetch posts

[Copons]

Changes proposed in this Pull Request

In #32529, the FSE blocks fetching posts logic was based upon an apiFetch fired from a custom made fetchPost async function.
While not wrong per se, it forced an API call regardless of the presence of such post in state, and on top of that, didn't take advantage of the Redux/withSelect perks.

• Remove the fetchPost utility, now unnecessary.
• Use select( 'core' ).getEntityRecord( 'postType', postType, postId ) instead of the more convoluted custom mix of getEntityRecord and apiFetch proposed in fetchPost.
• Make full use of HOC composition of withState and withSelect to request a new post (via getEntityRecord) only when a post has been selected and its ID and type have been stored as attribute, or into state (in the Content block, that doesn't persist it).

Testing instructions

• Smoke test Content and Template blocks:
  • Select a post and make sure the preview is displayed as expected.
  • Flip between edit and preview mode.
  • Save and reload, and check that Template shows up previewing the selected post. Note: this is currently broken because of Full Site Editing: Only allow FSE blocks in wp_template CPT #32657; a fix has been proposed in Full Site Editing: Conditionally enqueue blocks scripts when editing wp_template posts #32717).
• Make sure the blocks are saved as expected:
  • Content shouldn't persist anything.
  • Template should save the selected post ID and type as block attributes.

Fixes Dennis' concerns in #32529 (review) (well, at least tries to 😄)

[matticbot]

Test live: https://calypso.live/?branch=fix/fse-fetch-post

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[dmsnell]

Awesome! (I didn't test this)

[vindl]

LGTM!