Updates to table page objects

[mollykreis]

Pull Request

🤨 Rationale

Update the table page object to include functionality needed by some clients. Specifically,

• Add getHeaderTextContent to the page object so that clients don't have to access the header element to get the text content themselves
• Export TableColumnMappingPageObject from Angular
• Extends TablePageObject that's exported from Angular to include a new waitForDataUpdatesToRender to help clients wait for data to be rendered in the table when they are using the data$ property on the Angular table directive, which makes the timing of setData() outside of the client's control

👩‍💻 Implementation

Both the implementation of getHeaderTextContent and waitForDataUpdatesToRender were copied from the sl-table page object in SystemLinkShared.

🧪 Testing

• Updated a test in nimble-components to use the new nimble-components page object test
• Wrote new Angular tests to test the Angular page object

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@m-akinc, can you review buddy this PR for me?