Add support for specifying section header and footer titles

[simba909]

This adds support for specifying section header and footer titles by subclassing TableViewDiffableDataSource, just like the official version.

Checklist

• [x] All tests are passed.
• [ ] Added tests.
• [x] Documented the code using Xcode markup.
• [x] Searched existing pull requests for ensure not duplicated.

Description

This adds default implementations for tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) and tableView(_ tableView: UITableView, titleForFooterInSection section: Int), but leaving them as open functions, letting subclasses override their behaviour.

Motivation and Context

When UITableViewDiffableDataSource was introduced, setting section header and footer titles was not supported. Support for this has since been added, so I thought it would be nice if this library was updated to mirror the official version.

Impact on Existing Code

None

[ra1028]

@simba909 Thanks 👍