[Feature Request]: Add property to `ProjectRootElement` for convenient access to `<Sdk>` elements

[Zastai]

Summary

Project files can contain <Sdk> elements (modeled by ProjectSdkElement).
For parity with other project contents, there should be a property to access only those elements (e.g. SdkReferences).

Background and Motivation

There are already properties for accessing the Sdk attribute, all properties, all imports and all items.
However, while Sdk elements appear in Children, they have no dedicated convenience accessor of their own.

Proposed Feature

/// <summary>
/// Get a read-only collection of the SDK references.
/// </summary>
public ICollection<ProjectSdkElement> Sdks => new ReadOnlyCollection<ProjectSdkElement>(Children.OfType<ProjectSdkElement>());

(name could also be SdkElements for better distinction from Sdk; there is also ChooseElements)
(using Children and not AllChildren because I assume these elements are only valid at the top level)

A nice-to-have additional property (SdkReferences) or method (GetSdkReferences()) would be one that gathers all SDK references in a project, from all sources:

• Sdk attribute on the project
• Sdk elements
• Import elements that specify an SDK
  (That would make it easy for something like Dependabot to check whether a particular project references an SDK package for which an update is available.)

Alternative Designs

No response

[jrdodds]

Please assign this issue to me.

1. There is a clear request to modify Microsoft.Build.Construction.ProjectRootElement to add a new SdkElements property. (I'm favoring SdkElements over Sdks as the name.)

2. I have questions about the "nice-to-have additional property" that provides all SDKs. It seems like it should be on *.Evaluation.Project and not *.Construction.ProjectRootElement. An Import element that has both an Sdk attribute and a Condition attribute would be included only if the Condition evaluates to true. Does this have value? Should such a property be added or should the idea be tabled for discussion?

I can start on the first item.

Update: I have code changes for the first item ready for a PR. Because I don't want to create an unsolicited PR, I'll wait either for the issue to be assigned to me or for other feedback. Thanks

[jrdodds]

Please assign this issue to me. Thanks