Add GradientEmitter to support OMG-MEGA and OG-MAP-Elites #2084

Job	Run time
coverage	2m 1s
pin	3m 37s
benchmarks	2m 18s
examples	3m 16s
docs	2m 23s
pre-commit	1m 8s
test (ubuntu-latest, 3.11)	1m 43s
tutorials_list	4s
test (macos-latest, 3.11)	6m 51s
test (windows-latest, 3.11)	6m 7s
tutorials (tutorials/arm_repertoire.ipynb)	1m 52s
tutorials (tutorials/cma_mae.ipynb)	3m 57s
tutorials (tutorials/features/example.ipynb)	1m 17s
tutorials (tutorials/fooling_mnist.ipynb)	3m 1s
tutorials (tutorials/lsi_mnist.ipynb)	3m 37s
tutorials (tutorials/lunar_lander.ipynb)	2m 0s
tutorials (tutorials/tom_cruise_dqd.ipynb)	8m 13s
deploy	0s
	53m 25s

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