[wasm] Remove System.Reflection.MetadataLoadContext from WasmAppBuilder (#44751)

[wasm] Remove System.Reflection.MetadataLoadContext from WasmAppBuilder (#44751)

Separates the assembly and reference assembly loading process from the WasmAppBuilder into a separate task in WasmLoadAssembliesAndReferences.cs

Refactors that loading process to take in assemblies, search paths, and the option to skip missing referenced assemblies, thereby reducing the original complexity with respect to MainAssembly, ExtraAssemblies and having at least one of Assemblies or AssemblySearchPaths

Applies the updated WasmAppBuilder and WasmLoadAssembliesAndReferences tasks to the library tests (tests.mobile.targets), browser sample (WasmSample.csproj), debugger-tests (debugger-tests.csproj), and the runtime tests (WasmTestRunner.proj)

Co-authored-by: Mitchell Hwang <mitchell.hwang@microsoft.com>
Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>
Co-authored-by: Ankit Jain <radical@gmail.com>