Support explicit system-wide .NET runtime installation for extension clients

[JakeRadMSFT]

Motivation

C# Dev Kit Workspace Requirements discovers target framework requirements only after restore/design-time build and needs to remediate missing target runtime bands without installing a full SDK or using a private extension root.

Existing Behavior

Currently, local dotnet.acquire and global dotnet.acquireGlobalSDK exist, but there is no system-wide runtime/ASP.NET Core API.

Proposed API

We propose a new dotnet.acquireGlobalRuntime command accepting the existing IDotnetAcquireContext object:

• Required parameters: version, requestingExtensionId, installType: 'global', and a mode specifying either runtime or aspnetcore.
• Return type: IDotnetAcquireResult.

This is explicitly opt-in by the calling extension and does not change any default/local acquisition behaviors.

Compatibility & Security

• Preserve all existing APIs.
• Use official installers.
• Utilize existing elevation/hash/domain checks.
• Platform behavior is consistent with global SDK acquisition.

Acceptance Criteria

• Acceptance criteria for Windows/macOS/Linux where supported.
• Release metadata resolution for runtime and aspnetcore.
• Unit/integration tests.
• Command documentation and code sample.

I intend to update WIP PR #2573 to implement it.