Feat: Add border color option to button block

[hbhalodia]

What?

• This PR would add an option to add border color to buttons block. So user can change border color anytime for any buttons they use on the site.

Why?

Issue refrence - #43072

How?

• This PR changes the block.json file for button block and adds the support for border color to it.

Testing Instructions

1. Open any post/page.
2. Add Buttons block.
3. You can check the block controls option in sidebar having option to choose color for border.
4. Explicitly, you can also change the border width as this comes along with color option.

Testing Instructions for Keyboard

You can use Tabs to change color on newly option created.

Screenshots or screencast

[github-actions]

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[hbhalodia]

This is already resolved on trunk last week. Hence closing the PR.