Add experimental functionality for labelling PRs by their size #3849

Job	Run time
Build mode autobuild (ubuntu-latest, linked)	1m 39s
Build mode autobuild (windows-latest, linked)	3m 52s
Build mode autobuild (ubuntu-latest, nightly-latest)	1m 39s
Build mode autobuild (windows-latest, nightly-latest)	3m 37s
	10m 47s

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