Make it more ergonomic to make external types serializable

[aradalvand]

Today, if you have an external type (that you obviously can't annotate with [MemoryPackable]) that you want to serialize using MemoryPack, you fall off cliff where you basically have to resort to using a wrapper type + a custom formatter + register that formatter + annotate every property of that type with [MemoryPackAllowSerialize] wherever it appears.

This is clearly not the best developer experience, especially as contrasted with a simple [MemoryPackable] attribute.

I'd propose introducing an assembly-level attribute that allows users to instruct MemoryPack to generate a serializer for an external type + automatically register that in some static context using [ModuleInitializer] + have it be recognized as a memory-packable type everywhere (meaning no need for a gazillion [MemoryPackAllowSerialize]s everywhere — you can then use cutting-edge features like [UnsafeAccessor] (if necessary, to write to/invoke private properties/constructors) to source-generate serializers with no perf overhead.

Example:

[assembly: MemoryPackable<SomeExternalType>]

[aradalvand]

Update: Could potentially be closed in favor of #347 (if you think this is too much) which I think is a simpler/less controversial alternative.