TransformOutbound of route constraint is not called when the route segment is optional

### Describe the bug
TransformOutbound is not callled when generating url when the constraint in the route template is is optional.

### To Reproduce

https://github.com/vjacquet/bugs/blob/3fbc2924345021175b911b651b5afe3524cde361/aspnetcore/RouteConstraint/Startup.cs#L51

The Index page show that the last segment of the 2 urls are different when they shouldn't
Route.cshtml as the route definition @page "{id:hex}"
RouteWithOptionalSegment.cshtml as the route definition @page "{id:hex?}"

The problem is that `OptionalRouteConstraint`, that wrap the specified route contraint, does not implement `IOutboundParameterTransformer`, therefore [_parameterTransformers in TemplateBinder ](https://github.com/dotnet/aspnetcore/blob/1c126ab773059d6a5899fc29547cb86ed49c46bf/src/Http/Routing/src/Template/TemplateBinder.cs#L154) is not initialized correctly.

One possible fix is to define
```csharp
class OutboundParameterTransformerOptionalRouteConstraint : OptionalRouteConstraint, IOutboundParameterTransformer
{
public OutboundParameterTransformerOptionalRouteConstraint(IRouteConstraint innerConstraint) : base(innerConstraint)
{
}

public string TransformOutbound(object value) => ((IOutboundParameterTransformer)InnerConstraint).TransformOutbound(value);
}
```
and have the [`DefaultParameterPolicy`'s `InitializeRouteConstraint` method](https://github.com/dotnet/aspnetcore/blob/1c126ab773059d6a5899fc29547cb86ed49c46bf/src/Http/Routing/src/DefaultParameterPolicyFactory.cs#L68) return an instance of it when the constraint is optional and the routeConstraint implements `IOutboundParameterTransformer`.

```csharp
private IParameterPolicy InitializeRouteConstraint(bool optional, IRouteConstraint routeConstraint)
{
if (!optional)
return routeConstraint;
else if (routeConstraint is IOutboundParameterTransformer)
return new OutboundParameterTransformerOptionalRouteConstraint(routeConstraint);
else
return new OptionalRouteConstraint(routeConstraint);
}
```
### Further technical details
- ASP.NET Core version 3.1

.NET Core SDK (reflecting any global.json):
Version: 3.1.301
Commit: 7feb845744

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18362
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.301\

Host (useful for support):
Version: 3.1.5
Commit: 65cd789777

.NET Core SDKs installed:
1.1.14 [C:\Program Files\dotnet\sdk]
2.1.401 [C:\Program Files\dotnet\sdk]
2.1.802 [C:\Program Files\dotnet\sdk]
2.2.103 [C:\Program Files\dotnet\sdk]
3.1.200 [C:\Program Files\dotnet\sdk]
3.1.202 [C:\Program Files\dotnet\sdk]
3.1.301 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]