core: implement Kill/Automount/Mount Context/Runtime for io.systemd.Unit.List (#39391)

The PR implements the following objects + tests for
`io.systemd.Unit.List`:
- `KillContext`
- `AutomountContext`
- `AutomountRuntime`
- `MountContext`
- `MountRuntime`

It's a continuation of the following PRs:
* https://github.com/systemd/systemd/pull/37432
* https://github.com/systemd/systemd/pull/37646
* https://github.com/systemd/systemd/pull/38032
* https://github.com/systemd/systemd/pull/38212

src/basic/unit-def.c

@@ -72,6 +72,14 @@ const char* unit_dbus_interface_from_name(const char *name) {
         return unit_dbus_interface_from_type(t);
 }
 
+const char* unit_type_to_capitalized_string(UnitType t) {
+        const char *di = unit_dbus_interface_from_type(t);
+        if (!di)
+                return NULL;
+
+        return ASSERT_PTR(startswith(di, "org.freedesktop.systemd1."));
+}
+
 static const char* const unit_type_table[_UNIT_TYPE_MAX] = {
         [UNIT_SERVICE]   = "service",
         [UNIT_SOCKET]    = "socket",

src/basic/unit-def.h

@@ -321,6 +321,7 @@ void unit_types_list(void);
 
 DECLARE_STRING_TABLE_LOOKUP(unit_load_state, UnitLoadState);
 DECLARE_STRING_TABLE_LOOKUP(unit_active_state, UnitActiveState);
+const char* unit_type_to_capitalized_string(UnitType t);
 
 DECLARE_STRING_TABLE_LOOKUP(freezer_state, FreezerState);
 FreezerState freezer_state_finish(FreezerState state) _const_;