F multi retrieve

Summary

Implements multi-entity retrieval caosdb/customers/lfpb/management#393

See tests in caosdb-cppinttest!4 (merged)

Focus

• Most relevant changes are in include/caosdb/transaction.h and src/caosdb/transaction.cpp.
• New class MultiResultSet represents a result set with multiple entities. Is this a good name?
• New enum TransactionType restricts the kind of sub-transactions (Insert, Delete, Retrieve) which can be added to a transaction.
• Macros are used to check whether InsertEntity, DeleteById or RetrieveById can be called on a transaction (given the TransactionStatus and 'TransactionType`).

Test Environment

Test with server in branch f-grpc-f-multi-retrieve

Check List for the Author

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• All automated tests pass
• Reference related Issues
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  • Intent of new code
  • Problems with old code
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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?

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include/caosdb/entity.h

@@ -319,9 +319,10 @@ public:
   [[nodiscard]] inline auto HasErrors() const -> bool {
     return this->errors.wrapped->size() > 0;
   }
-  // TODO(fspreck) These two need implementations...
-  [[nodiscard]] auto GetWarnings() const -> const Messages &;
-  [[nodiscard]] auto GetInfos() const -> const Messages &;
+  [[nodiscard]] auto GetWarnings() const -> const Messages & {
+    return warnings;
+  }
+  [[nodiscard]] auto GetInfos() const -> const Messages & { return infos; }
 
   inline auto ToString() const -> const std::string {
     google::protobuf::util::JsonOptions options;