feat(ui5-button): introduce support for form attribute

[didip1000]

This PR introduces the form property to ui5-button, enabling buttons to submit forms even when placed outside the form element. This brings the component in line with native HTML button behavior.

Usage:

The form property in the ui5-button takes in an ID of a form as a string allowing the button to submit the associated form:

<form id="externalForm">
    <!-- form fields -->
</form>

<ui5-button form="externalForm" type="Submit">Submit</ui5-button>

Fixes: #7459

[ui5-webcomponents-bot]

🧹 Preview deployment cleaned up: https://pr-13321--ui5-webcomponents.netlify.app

[ui5-webcomponents-bot]

🎉 This PR is included in version v2.21.0 🎉

The release is available on v2.21.0

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