Lumin Support

**Context**

We have a Unity project that utilizes the Lumin build engine. This can be done all well and good in the Unity UI (and in CLI too!) but `unity-builder` doesn't support Lumin builds. Is there a chance that this package could host this kind of support? Is this on the roadmap?

**Suggested solution**

1. Project requires a `.privkey` and `.cert` in the project hierachy
2. `... \2021.1.19f1\Editor\Unity.exe" -quit -batchmode -executeMethod BuildScript.BuildLumin-logFile ".\Logs\lumin-build.log"`

*Assets/Editor/BuildScript.cs*
```cs

static void BuildLumin(){
Directory.CreateDirectory("dist/Lumin");

string[] defaultScene = {
"Assets/SimplyVideo/Demo/Demo.unity"
};

BuildPipeline.BuildPlayer(
defaultScene,
$"dist/Lumin/MagicLeap-{UnityEngine.Application.version}.mpk" ,
BuildTarget.Lumin,
BuildOptions.None
);
}
```
The above build script builds a Lumin project. I'm not exactly sure how this fits into the hierarchy of the `unity-builder project, but you could run the locally (with Lumin SDK installed correctly).
**Considered alternatives**

**Additional details**

The issue names Assets/Editor/BuildScript.cs and the Unity.exe command using BuildLumin and BuildTarget.Lumin. Start by comparing that example with unity-builder's existing platform-build entry points, then determine how the .privkey and .cert requirements fit. Done means Lumin builds work through the package with the required credentials and are covered by appropriate validation.

Written by the indexing model from the issue text.