Support async validation in minimal APIs

[captainsafia]

We should react to dotnet/runtime#121536 one the APIs to support async validation are supported.

[danroth27]

@SamMonoRT I assume this should go to @halter73 at this point.

[Digvijay]

I've been exploring this space with an experimental, open-source validation source generator called Sannr. It's not a competitor to Microsoft.Extensions.Validation — it's a personal research project building in the open.

Async validation is supported today in Sannr:

public partial class UserRegistration
{
    [CustomValidator(typeof(UserRules), nameof(UserRules.IsUniqueAsync), IsAsync = true)]
    public string Email { get; set; }
}

public static class UserRules
{
    public static async Task<ValidationResult> IsUniqueAsync(string email, IServiceProvider sp)
    {
        var db = sp.GetRequiredService<AppDbContext>();
        bool exists = await db.Users.AnyAsync(u => u.Email == email);
        return exists 
            ? ValidationResult.Error("Email is already taken.") 
            : ValidationResult.Success();
    }
}

[Youssef1313]

Related: dotnet/designs#363