Update EventMergeRequest webhook to handle non-string merge param values

[klew00]

Calls from GitLab's merge request webhook event can have the merge_params property contain non-string values, such as should_remove_source_branch which is a boolean or train_ref which is an object (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/129979). Example:

"object_attributes": {
  ...
  "merge_params": {
    "force_remove_source_branch": "1",
    "auto_merge_strategy": "merge_train",
    "should_remove_source_branch": true,
    "sha": "61f4798ee6668860a47f2f84b31bf8b88ab52bb9",
    "train_ref": {
      "commit_sha": "83d69a142f2fb301c3cce0a802ec1c5ee3a091a8",
      "merge_commit_sha": "83d69a142f2fb301c3cce0a802ec1c5ee3a091a8",
      "squash_commit_sha": "61f4798ee6668860a47f2f84b31bf8b88ab52bb9"
    }
  },
  ...
}

This causes issues when handling the event since the merge_params attribute in EventMergeRequest is a Map<String, String>, which expects every value to be a string. This change updates merge_params so it can handle the should_remove_source_branch and train_ref non-string values when converting the JSON.