Add type-bound MemoryPackCustomFormatter

[aradalvand]

Currently, if you have a custom formatter for a particular type (e.g. NodaTime's Instant, for example), you have to specify the formatter on every single property of that type — this is tedious and error-prone:

public record SomeDto(
    // ...
    [property: InstantFormatter]
    Instant CreatedAt,
    [property: InstantFormatter]
    Instant UpdatedAt,
    [property: InstantFormatter]
    Instant ExpiresAt
    // ...
);

public class InstantFormatter : MemoryPackCustomFormatterAttribute<Instant>
{
    public override IMemoryPackFormatter<Instant> GetFormatter() => new Formatter();

    private class Formatter : MemoryPackFormatter<Instant>
    {
        public override void Serialize<TBufferWriter>(ref MemoryPackWriter<TBufferWriter> writer, scoped ref Instant value)
        {
            writer.WriteValue(value.ToUnixTimeTicks());
        }

        public override void Deserialize(ref MemoryPackReader reader, scoped ref Instant value)
        {
            value = Instant.FromUnixTimeTicks(reader.ReadValue<long>());
        }
    }
}

Introduce a way to globally associate a type with a specific formatter — e.g. using an assembly attribute:

[assembly: MemoryPackCustomFormatter<Instant, InstantFormatter>]

Additionally, add a way to annotate a type directly with a custom formatter (a la [JsonConverterAttribute]).

[MemoryPackCustomFormatter<FooFormatter>]
public readonly struct Foo { ... }

So as to eliminate the need for individual annotations on every single property of that type. This is highly useful in cases where there's a 1:1 correspondence between a formatter and a type (which is extremely common).

[aradalvand]

@neuecc Hello, would you accept a PR for this?

[aradalvand]

This (specifically the last snippet in my original comment) would help tools like Vogen to add MemoryPack support.

[aradalvand]

@neuecc Any thoughts? MemoryPack effectively lacks proper extensibility points without this.