Distribution: Add support for external pid as group leader

[pguyot]

Continuation of #1468

• Store group leader pid on the heap if it's an external pid
• Introduce a specific signal to change group leader to an external pid as we cannot touch heap from another process
• Add simple test based on rpc and BEAM

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later