Post Content: Add border and spacing support

[rinkalpagdar]

What?

Add border block support to the post content block.
Part of #43247
Part of #43243

Why?

The post content block is missing border support.

How?

Adds the border block support in block.json

Testing Instructions

1. Go to the Global Styles setting ( under appearance > editor > styles > edit styles > blocks )
2. Make sure that the content block's border is configurable via Global Styles
3. Verify that Global Styles are applied correctly in the editor and frontend
4. Edit the post, Add content block and Apply the border styles
5. Verify that block styles take precedence over global styles
6. Verify that block borders display correctly in both the editor and frontend

Screenshots or screencast

[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: rinkalpagdar <[email protected]>
Co-authored-by: tellthemachines <[email protected]>

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

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @rinkalpagdar! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[tellthemachines]

Thanks for the PR! It's working well for me in testing. Could you just revert the whitespace changes in block.json?

[rinkalpagdar]

@tellthemachines Thanks for the feedback. I have reverted the spacing in block.json.

[tellthemachines]

Thanks for the update, LGTM now!