Add is_blocked_by option for issue links API

What does this MR do and why?

Issue: #365796 (closed)

We already mentioned in the docs that is_blocked_by is supported for the Issue Link API, but we rejected the type. Now we support the is_blocked_by link type for the Issue Link API: :id/issues/:issue_iid/links.

Screenshots or screen recordings

(Note, the screenshot shows the wrong ID in my terminal as I played around with the API before)

How to set up and validate locally

1. Create two issues in a project
2. Execute the following curl command with your Private Access Token for your local instance to set the type is_blocked_by for Issue 1:

curl --header "PRIVATE-TOKEN: $TOKEN" -X POST "http://gdk.test:3000/api/v4/projects/$PROJECT_ID/issues/$SOURCE_ISSUE_IID/links?target_project_id=$PROJECT_ID&target_issue_iid=$TARGET_ISSUE_IID&link_type=is_blocked_by" | jq

3. The target_issue then blocks the issue with the iid 1

MR acceptance checklist

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