[extension/awsmiddleware] Add AWS middleware extension interface (#116) #512

Job	Run time
windows-unittest-matrix (receiver-0)	29m 18s
windows-unittest-matrix (receiver-1)	29m 12s
windows-unittest-matrix (processor)	19m 9s
windows-unittest-matrix (exporter)	33m 17s
windows-unittest-matrix (extension)	18m 30s
windows-unittest-matrix (internal)	17m 48s
windows-unittest-matrix (pkg)	16m 18s
windows-unittest-matrix (cmd)	33m 55s
windows-unittest-matrix (other)	35m 58s
windows-unittest	6s
	3h 53m 31s

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