Pass issuable-type in AddIssuableForm

What does this MR do?

This MR adds a new prop to be sent down to the AddIssuableForm. This prop is called issuableType and is mainly used for display purposes in the input field.

What are the relevant issue numbers?

#9125 (closed) #9077 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #9125 (closed); Closes #9077 (closed)