Feat: more powerful Attribute-based configuration

Nobody has claimed this yet.

It would be nice if we could use attributes to define any (or most...) aspects of mapping -- and if we could derive our own attributes to do that.

### Use case
I want to be able to easily say, with an attribute, that a property is mapped only if another property has a specific value.

If I was doing this in code, it would be:
```cs
config.Map(target => target.Prop, src => src.Prop, src => src.Other == SentinelValue);
```

Instead, I wrote an attribute to configure this aspect:
```cs
class SourceDto
{
public string Other { get; set; }
[AdaptNullUnless(nameof(Other), SentinelValue)]
public string Prop { get; set; }
}
```

Of course, because this attribute is unknown to Mapster, I also had to create an `IRegister` implementation that configures those mappings automatically using reflection.

It would be nice if that last part was not required.

### Suggestion
Similar to other frameworks that use Attributes as an alternative configuration (I'm thinking Grace IoC here), maybe Mapster could provide a base attribute class:

```cs
// Provided my Mapster
abstract class MapsterPropertyAttribute : Attribute
{
public void abstract Map(PropertyInfo property, TypeAdapterSetter config);
}
```

Maspter would then find any attribute derived from `MapsterPropertyAttribute` when building a mapping and invoke `Map` on them.

With this, users could build their own attributes and apply them without any other discovery step.

My example use case would be entierly contained in this attribute:
```cs
public class AdaptNullUnlessAttribute : MapsterPropertyAttribute
{
private string other;
private object value;

public AdaptNullUnlessAttribute(string other, object value)
{
this.other = other;
this.value = value;
}

public override void Map(PropertyInfo prop, TypeAdapterSetter config)
{
config.Settings.Resolvers.Add(new InvokerModel
{
// This assumes same name, maybe Mapster should pass the config so far to this method,
// esp. the target name if AdaptMember was also used.
DestinationMemberName = p.Name,
SourceMemberName = p.Name,
Condition = Expression.Lambda(...), // Removed for conciseness
});
}
}
```

The issue names no repository files or tests. Start by tracing existing attribute handling and IRegister configuration around TypeAdapterSetter, PropertyInfo, and InvokerModel; done would mean a derived attribute can be discovered and affect a mapping without a separate registration step.

Written by the indexing model from the issue text.