List updated packages in codeflow PRs

[premun]

Context

As part of #3364, we started generating better PR descriptions.
However, they do not yet contain the list of updated packages.
Furthermore, with #4342, we will support adding, updating and removal of packages which the list should reflect correctly.

Goal

List the package changes in codeflow PR descriptions. The listed version diffs should link to the real diff in code just like in the current Maestro PRs.

[dkurepa]

Not sure if it's possible, but we might be able to reuse the same logic for this as the Dependency Flow updates

[adamzip]

From a discussion with Premek, I believe as part of this task we can also do updates of the PR title.
The title could depend on whether the PR only brings source code changes, or only updates, or both.
Eg:

1. [main] Update dependencies from dotnet/dotnet
2. [main] Update code from dotnet/dotnet
3. [main] Update code & dependencies from dotnet/dotnet