feat(ui5-user-menu): add support for custom footer in user menu

[dobromiraboycheva]

Overview

Introducing a new footer slot to the UserMenu component, allowing applications to provide custom footer content instead of the default "Sign Out" button.

Structure

• When no footer slot is provided, the default "Sign Out" button is displayed
• When a footer slot with content is provided, the custom content replaces the default footer
• When an empty footer slot is provided (<div slot="footer"></div>), the footer area is completely hidden

API

UserMenu new slot:

Slot	Description
footer	Defines custom footer content. When provided, replaces the default "Sign Out" button.

Usage

Default footer (Sign Out button):

<ui5-user-menu>
  <ui5-user-menu-account slot="accounts" title-text="John Doe"></ui5-user-menu-account>
  <ui5-user-menu-item icon="action-settings" text="Setting" data-id="setting"></ui5-user-menu-item>
</ui5-user-menu>

Custom footer:

<ui5-user-menu>
  <ui5-user-menu-account slot="accounts" title-text="John Doe"></ui5-user-menu-account>
  <ui5-user-menu-item icon="action-settings" text="Setting" data-id="setting"></ui5-user-menu-item>
  <div slot="footer">
    <ui5-button icon="add" design="Transparent"></ui5-button>
    <ui5-button icon="log" design="Transparent">Sign Out</ui5-button>
  </div>
</ui5-user-menu>

Hidden footer:

<ui5-user-menu>
  <ui5-user-menu-account slot="accounts" title-text="John Doe"></ui5-user-menu-account>
  <ui5-user-menu-item icon="action-settings" text="Setting" data-id="setting"></ui5-user-menu-item>
  <div slot="footer"></div>
</ui5-user-menu>

Fixes: #12941
Fixes: #12942

[ui5-webcomponents-bot]

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