core/api: Add missing LICENSE files to sub-modules (#27920)

Go module tooling mandates that each sub-module contains its own LICENSE file.
If absent, it defaults to the LICENSE file in the root of the git repository.
This resulted in the api/auth/* modules erroneously inheriting the BUSL
license instead of the correct MPL license, as indicated by the SPDX info in
the actual API code.

This update ensures that module documentation is displayed correctly on
pkg.go.dev and resolves issues with various tools showing incorrect license
information for the sub-modules.

Signed-off-by: Tero Saarni <[email protected]>

Author: tsaarni

api/auth/approle/LICENSE

@@ -0,0 +1,3 @@
+```release-note:bug
+core/api: Added missing LICENSE files to API sub-modules to ensure Go module tooling recognizes MPL-2.0 license.
+```