Provide trimming annotations for self-contained apps

[yaakov-h]

.NET 5 previewed the ability to trim unused code from applications and from the runtime, and .NET 6 now makes that generally available.

Relevant docs are here: https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/trim-self-contained

Considering how many protobuf messages we ship that most apps won't use, this is quite valuable. DepotDownloader can trim about 4.5-5 MB away from SteamKit2.dll and a fully self-contained trimmed build of DD is about 15-20MB IIRC.

Related issues:

• .NET 6: Trimmer trims collection item objects away dotnet/linker#2365 (blocking)
• .NET 6: Trimmer does not follow async flows dotnet/linker#2366 (non-blocking)
• Add NestedTypes to DynamicallyAccessedMember use protobuf-net/protobuf-net#817 (probably non-blocking)

Some initial prototyping has already been done in this pull request:

• .NET 6.0 experimentation #1002

[yaakov-h]

We may have to switch to the Google.Protobuf library which isn't reflection-based in order to (a) get this working properly, and (b) be able to run in an AOT environment such as what is being planned at the moment for .NET 7.

🤔