[ResourceList] Add headerContent prop for customizing header on ResourceList

[krithikajanakiraman]

WHY are these changes introduced?

Related issue https://github.com/Shopify/ecosystem-merchant-engagement/issues/1106
Currently there is no way to show a customized header on the resource list and the only options available are 'Showing x items' or 'Showing x of y items'.

As part of the updated app settings project on admin, we wanted to show some other content and got in touch with the polaris team and learnt we could contribute since this doesn't exist at the moment.

WHAT is this pull request doing?

This PR adds an optional headerContent prop of type string which lets the user pass in any heading they want to display on the resource list header.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[chloerice]

Thanks for contributing @krithikajanakiraman! Tophat and code look good to me 👍🏽

You just need to also:

• Sign the Shopify CLA (see the failing check's "Details" button)
• Add the "With header content" example to the ResourceList component's documentation examples by adding a resource-list-with-header-content.tsx file to the polaris/polaris.shopify.com/pages/examples directory 😁