collection: Add SPDX headers to all files of the collection

[berndfinger]

We discussed among contributors if we want SPDX headers, and voted unanimously for adding a one-line SPDX header to all the files of the collection.

See SPDX short form identifier in https://spdx.dev/learn/handling-license-info/ for examples and more information.

[Wabri]

Sorry @berndfinger I didn't see that you already assigned yourself! Do you want me to close that pr?

[berndfinger]

Solved in #778 . Thanks for this @Wabri !