Backport: add support for label element on the elements API

[sarthaknagoshe2002]

Backport for ​https://github.com/WordPress/gutenberg/pull/81160

What?

Part of ​https://github.com/WordPress/gutenberg/issues/34198
This PR adds elements support to the label element.

Why?

So a theme can consistently style how form labels look across the site, regardless of the blocks or third party plugins used. This continues the form elements work recently done for inputs and selects.

How?

By allowing styling of label using the elements API

Testing Instructions

1. Add the following to theme.json file:

"styles": {
		"elements": {
			"label": {
				"color": {
					"text": "red",
					"background": "blue"
				},
				"spacing": {
					"padding": {
						"top": "25px",
						"bottom": "25px",
						"left": "25px",
						"right": "25px"
					}
				}
			}
		}
	},

2. Insert a Custom HTML block with <label>Test Label</label> and verify the colors applied correctly in the editor and on the frontend.

Screenshots or screencast

Trunk	This PR with theme.json changes	This PR with no theme.json changes

Fixes ​https://github.com/WordPress/gutenberg/pull/81647.

Trac ticket: https://core.trac.wordpress.org/ticket/65899

Backport for: ​https://github.com/WordPress/gutenberg/pull/81160

[wildworks]

Since the pull request ​https://github.com/WordPress/gutenberg/pull/81160 has not yet been merged, I believe the milestone should be 7.2, not 7.1.