ENH: Implement queries and entity retrieval

Summary

See https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/386.

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 spezifications? Are they satisfied?

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

docs/src/api.md

@@ -5,16 +5,24 @@ interface. You also find the documentation of the internal API which
 is meant for expert use only. Only use those if you know what you're
 doing.
 
+```@index
+Order = [:module, :function, :macro, :type, :constant]
+```
+
 ## Public API
 
 ```@autodocs
-Modules=[CaosDB, CaosDB.Exceptions, CaosDB.Info, CaosDB.Utility, CaosDB.Connection, CaosDB.Authentication, CaosDB.Entity]
+Modules=[CaosDB, CaosDB.Exceptions, CaosDB.Info, CaosDB.Utility, 
+	CaosDB.Connection, CaosDB.Authentication, CaosDB.Entity, 
+	CaosDB.Transaction]
 Private=false
 ```
 
 ## Expert-use only API functions
 
 ```@autodocs
-Modules=[CaosDB, CaosDB.Exceptions, CaosDB.Info, CaosDB.Utility, CaosDB.Connection, CaosDB.Authentication, CaosDB.Entity]
+Modules=[CaosDB, CaosDB.Exceptions, CaosDB.Info, CaosDB.Utility, 
+	CaosDB.Connection, CaosDB.Authentication, CaosDB.Entity, 
+	CaosDB.Transaction]
 Public=false
 ```