[Suggestion] Add options for specifying external type proxies on class fields

[Deficuet]

Currently, the proxies for an external type are not picked automatically for class/struct fields

using System.Numerics;

[GenerateSerde(ForType = typeof(Vector2))]
public partial class Vector2Proxy;

[GenerateSerde]
public partial class Test
{
    public Vector2 vec;     // Serde.ERR_DoesntImplementInterface
}

This can be solved by explicitly state proxy via SerdeMemberOptions

[GenerateSerde]
public partial class Test
{
    [SerdeMemberOptions(Proxy = typeof(Vector2Proxy))]
    public Vector2 vec;     // ok
}

But the description inflates rapidly as the type becomes complex

[GenerateSerde]
public partial class Test
{
    [SerdeMemberOptions(
        SerializeProxy = typeof(
            ArrayProxy.Ser<Vector2[], ArrayProxy.Ser<Vector2, Vector2Proxy>>
        ),
        DeserializeProxy = typeof(
            ArrayProxy.De<Vector2[], ArrayProxy.De<Vector2, Vector2Proxy>>
        )
    )]
    public Vector2[][] vec;     // ok
}

I think this is too tedious because the only unknown part is Vector2Proxy
So I hereby suggest to add or extend attribute for specifying external type proxy.

Conceptual design:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = true, Inherited = true)]
#if !SRCGEN
public
#else
internal
#endif
sealed class UseProxy : Attribute
{
    public required Type ForType { get; init; }
    public required Type Proxy { get; init; }
    public SerdeUsage Usage { get; init; } = SerdeUsage.Both;    // make enum SerdeUsage public?
}

[GenerateSerde]
[UseProxy(ForType = typeof(Vector2), Proxy = typeof(Vector2Proxy))]
[UseProxy(ForType = typeof(Vector3), Proxy = typeof(Vector3Proxy2), Usage = SerdeUsage.Serialize)]
[UseProxy(ForType = typeof(Vector3), Proxy = typeof(Vector3Proxy), Usage = SerdeUsage.Deserialize)]
public partial class Test
{
    public Vector3 vec1;
    public Vector2[][] vec;
    [UseProxy(ForType = typeof(Vector3), Proxy = typeof(AnotherVector3Proxy))]
    public MapPair<Vector3, Vector2[]> vec3;
}

The attributes applied on fields have higher priority and can override the one applied on class. This also could be a replacement of proxies properties currently in member options.

FYI:

In Kotlin standard serialization library, the file-scoped annotation @UseSerializers is used to solve such problem at compile time, and SerializersModule can do similar thing but at runtime.

[Deficuet]

I developed a prototype on my fork. Ser/De Proxy peoperties in SerdeMemberOptions are not touched but I think UseProxy will be a replacement of them.

[Deficuet]

Actually it can't handle the scenario when multiple proxies are required for one external type in a field, e.g.

[GenerateSerde]
public partial class Test
{
    [SerdeMemberOptions(SerializeProxy = typeof(
        MapPairProxy.Ser<Vector3, Vector3, V3Proxy1, V3Proxy2>
    ))]
    public required MapPair<Vector3, Vector3> pair;
}