Add basic implementation for APP-6 SIDCs.

These will be used in conjunction with
https://github.com/spatialillusions/milsymbol to simplify the
specification of unit type to the front-end.

tests/test_sidc.py

@@ -0,0 +1,17 @@
+from game.sidc import (
+    LandInstallationEntity,
+    StandardIdentity,
+    Status,
+    SymbolIdentificationCode,
+    SymbolSet,
+)
+
+
+def test_sidc() -> None:
+    sidc = SymbolIdentificationCode(
+        standard_identity=StandardIdentity.FRIEND,
+        symbol_set=SymbolSet.LAND_INSTALLATIONS,
+        status=Status.PRESENT_DAMAGED,
+        entity=LandInstallationEntity.AIPORT_AIR_BASE,
+    )
+    assert str(sidc) == "10032030001213010000"