Consolidate labels widget architecture

What does this MR do and why?

• Update issuable labels from within widget instead of doing it from the parent. Mutation is simplified using labelIds instead of addLabelIds.
• Make attrWorkspacePath and labelType props required to simplify mutation variable to create a label.
• Remove unnecessary selectedLabels prop from labels_select_root as issuable labels are already fetched within the widget.

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Related to #342970 (closed)