core: Add ExecSearchPath parameter to specify the directory relative to which binaries executed by Exec*= should be found

Currently there does not exist a way to specify a path relative to which
all binaries executed by Exec should be found. The only way is to
specify the absolute path.

This change implements the functionality to specify a path relative to which
binaries executed by Exec*= can be found.

Closes #6308

src/shared/bus-unit-util.c

@@ -969,6 +969,7 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                               "InaccessiblePaths",
                               "ExecPaths",
                               "NoExecPaths",
+                              "ExecSearchPath",
                               "RuntimeDirectory",
                               "StateDirectory",
                               "CacheDirectory",