Support passing game.start_command to mods

This change together with praydog/uevr-frontend#9 will allow one click 'launch to VR' from within Rai Pal.
This also introduces proper division of responsibility where Rai Pal knows what to launch and UEVRInjector knows how to launch it.

On top of the code change UEVR `rai-pal-manifest.json` needs to look like this:
```json
{
  "version": "1.03",
  "runnable": {
    "path": "UEVRInjector.exe",
    "args": [
      "--attach={{ExecutableName}}", "--delay=30", "--launch={{StartCommand}}"
    ]
  },
  "engine": "Unreal",
  "unityBackend": null
}
```

backend/src/mod_loaders/runnable_loader.rs

@@ -27,7 +27,7 @@ use crate::{
 	result::Error,
 	serializable_enum,
 	serializable_struct,
-	Result,
+	Result, providers::provider_command::ProviderCommand,
 };
 
 serializable_struct!(RunnableLoader {
@@ -38,6 +38,7 @@ serializable_enum!(RunnableParameter {
 	ExecutableName,
 	ExecutablePath,
 	GameJson,
+	StartCommand,
 });
 
 #[async_trait]
@@ -92,6 +93,15 @@ fn replace_parameters(argument: &str, game: &InstalledGame) -> String {
 	result = replace_parameter_value(&result, RunnableParameter::GameJson, || {
 		Ok(serde_json::to_string(&game)?)
 	});
+	result = replace_parameter_value(&result, RunnableParameter::StartCommand, || {
+		match &game.start_command {
+			Some(provider_command) => match provider_command {
+				ProviderCommand::String(s) => Ok(s.to_string()),
+				ProviderCommand::Path(_, _) => Err(Error::AppInfoNotFound(String::from("Provider command not found")))
+			}
+			None => Err(Error::AppInfoNotFound(String::from("Provider command not found")))
+		}
+	});
 
 	result
 }