Switch to dotnet-watch delta appliers

[tmat]

Microsoft Reviewers: Open in CodeFlow

Replaces the use of default DeltaApplier implemented in WebTools with one from dotnet-watch (DefaultHotReloadClient type shared via Microsoft.DotNet.HotReload.Client source package).
The Hot Reload client is wrapped in DeltaApplier type that implements the existing IDeltaApplier interface.

Adds a new interface IProjectHotReloadSessionWebAssemblyCallback for Web Project to implement. Once implemented the Hot Reload session implementation will switch to using dotnet-watch WebAssemblyHotReloadClient as the underlying implementation of the DeltaApplier for WASM app.

Adds AbstractBrowserRefreshServerAccessor that exposes Browser Refresh server implementation from Microsoft.DotNet.HotReload.Client source package to WebTools, so that we can replace the implementation in WebTools.

Adds Browser Refresh middleware (package Microsoft.DotNet.HotReload.Web.Middleware published by dotnet/sdk) to Managed Project System VSIX to distribute with VS. Once we switch WebTools to AbstractBrowserRefreshServerAccessor it will stop using the middleware from the SDK and switch to the middleware included in Project System VSIX. This will allow us to version the client (IDE) and the middleware together instead of having to support all combinations of VS and SDK versions.

Contributes to
https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2398434
https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2398433
https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2398432

[tmat]

@LittleLittleCloud @drewnoakes ptal

[drewnoakes]

LGTM though will defer to @LittleLittleCloud for final sign off.