ENH: Allow insert/update/delete and files in Extern C

Summary

See https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/412

Focus

Point the reviewer to the core of the code change. Where should they start
reading? What should they focus on (e.g. security, performance,
maintainability, user-friendliness, compliance with the specs, finding more
corner cases, concrete questions)?

Test Environment

How to set up a test environment for manual testing?

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.

include/caosdb/file_transmission/download_request_handler.h

@@ -68,6 +68,9 @@ using caosdb::entity::v1alpha1::FileTransmissionService;
 using caosdb::transaction::HandlerInterface;
 using caosdb::transaction::HandlerTag;
 
+/*
+ * Handler for the download request of a single file
+ */
 class DownloadRequestHandler final : public HandlerInterface {
 public:
   DownloadRequestHandler(HandlerTag tag, FileTransmissionService::Stub *stub,