feat: add support for resolving PostContent blocks

[justlevine]

What

This PR adds support for resolving innerBlocks for core/post-content blocks.

Important

This PR is based off of #324, which should be merged first.
Relevant diff: 8e2e921

Why

This is essential for supporting FSE, and due to the final/private nature of ContentBlocksResolver cannot be easily hooked in from external code.

While the WPGraphQL core does not (yet) support querying for a complete FSE template (where core/post-content is traditionally used, I believe the above consideration along with the following reasons still make this a good candidate for inclusion:

1. While an unlikely edge case, the fact that it's replicable in the tests using public hooks and functions means it not premature for inclusion.
2. It has the added benefit of giving us a real-world test case for the wpgraphql_content_blocks_resolver_content, that goes along with validating the @todo in the Post model gate
3. As with the two proceeding PRs, this is a backwards-compatible enhancement (also future compatible with FSE), so it makes sense to add it before a pending breaking release.

*Note: This PR is based on #324 solely to prevent merge conflicts. There are no code dependencies between the commits.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5381c84

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@wpengine/wp-graphql-content-blocks	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[justlevine]

Even basing on another branch didn't seem to prevent the merge conflict 🤦

Rebased