Post-content block header color support

[shimotmk]

What?

Similar #49131

Related:

Tracking: Addressing Design Tooling Consistency #43241
Tracking: Color Design Tools Consistency #43245

Why?

Post-content can change the color for each header in theme.json.
I think post-content header color should be supported.

How?

Testing Instructions

1. Open site editor
2. Edit post content block
3. You can edit post content heading color

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

Tag
[Type] Enhancement
[Feature] Design Tools
[Block] Post Content

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: shimotmk <[email protected]>
Co-authored-by: aaronrobertshaw <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[shimotmk]

@aaronrobertshaw
Can you please leave a review?

[aaronrobertshaw]

Thanks for the PR @shimotmk 👍

✅ Heading color controls appear for the Post Content block
✅ Global Styles for Post Content heading colors apply in editor and frontend
✅ Block instance styles on heading blocks override Post Content heading colors

LGTM 🚢

[shimotmk]

@aaronrobertshaw
Thanks for your review! Is there anything else I need to do?

[aaronrobertshaw]

There was a failed e2e test around labels. I've re-run that and taken the liberty to merge this for you.

[shimotmk]

Thank you for merge!