twister: add support for custom emulator in simulator

Enable the possibility for boards to implement a custom `run` target in
its board.cmake to run any arbitrary commands. This is helpful for devs
who would like to add support for proprietary simulator to their boards
that can't be upstreamed.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

scripts/schemas/twister/platform-schema.yaml

@@ -24,7 +24,19 @@ mapping:
     enum: ["mcu", "qemu", "sim", "unit", "native"]
   "simulation":
     type: str
-    enum: ["qemu", "simics", "xt-sim", "renode", "nsim", "mdb-nsim", "tsim", "armfvp", "native"]
+    enum:
+      [
+        "qemu",
+        "simics",
+        "xt-sim",
+        "renode",
+        "nsim",
+        "mdb-nsim",
+        "tsim",
+        "armfvp",
+        "native",
+        "custom",
+      ]
   "simulation_exec":
     type: str
   "arch":