Enhance issue handling by adding utils module

Introduce `append_related_issues` function and `IssueRelated` enum to manage related issue linking. Update Gitea action to utilize the new functionality for appending related issues in work package content. Add corresponding tests.

src/planning/mod.rs

@@ -4,6 +4,7 @@ use crate::error::GeneralError;
 use url::Url;
 
 pub(crate) mod issue2work;
+pub mod utils;
 
 pub trait Issue2WorkActionTrait {
     async fn run(&self, url: &Url, config: &Config, args: &Issue2Work) -> Result<(), GeneralError>;