Add more options to control trimming behavior

https://github.com/mono/linker/issues/1269#issuecomment-731231382 proposes two new ways to configure trimming:
- Allow opting into trimming via an ItemGroup, not just via IsTrimmable metadata
- A property that simplifies opting everything into trimming: TrimAllAssemblies

Currently, the `IsTrimmable` metadata in MSBuild controls which assemblies are trimmed (see https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming-options#trimmed-assemblies). The metadata can be set on a `PackageReference` and it will flow to the referenced assemblies, but setting it for individual assemblies requires adding this metadata at the appropriate step in the build with a custom target. The above suggestions would simplify this. For example:

```xml

true

```
could be simplified to:
```xml

```

It would be important to clarify whether the ItemGroup refers to assemblies by name or by file path (it's more common for ItemGroups to represent files on disk). This is similar to the behavior of [`TrimmerRootAssembly`](https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming-options#root-assemblies).