Add SecAggPlus middleware #6583

Job	Run time
Build, test and upload wheel	1m 19s
Framework / bare	2m 45s
Framework / bare-https	1m 15s
Strategy / FedMedian	1m 13s
Framework / jax	2m 5s
Strategy / FedTrimmedAvg	1m 19s
Framework / pytorch	2m 45s
Strategy / QFedAvg	1m 8s
Framework / tensorflow	3m 10s
Strategy / FaultTolerantFedAvg	1m 11s
Framework / tabnet	2m 40s
Strategy / FedAvgM	1m 7s
Framework / opacus	2m 46s
Strategy / FedAdam	1m 17s
Framework / pytorch-lightning	2m 31s
Strategy / FedAdagrad	1m 14s
Framework / scikit-learn	2m 23s
Strategy / FedYogi	1m 8s
Framework / fastai	3m 53s
Framework / pandas	1m 40s
	38m 49s

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