Add `budget` field to experiment spec #901

Job	Run time
Compatibility	37s
Docker	18s
ubuntu-latest - python 3.8 - Lint	36s
Pydantic V1	46s
ubuntu-latest - python 3.8 - Type check	35s
ubuntu-latest - python 3.8 - Unit tests	1m 38s
ubuntu-latest - python 3.8 - Doc tests	37s
ubuntu-latest - python 3.8 - Doc tests (B)	33s
ubuntu-latest - python 3.8 - Images	1m 1s
ubuntu-latest - python 3.8 - Jobs	56s
ubuntu-latest - python 3.8 - Experiments	1m 0s
ubuntu-latest - python 3.8 - Datasets	1m 38s
ubuntu-latest - python 3.8 - Sweep example	1m 32s
ubuntu-latest - python 3.8 - Build	42s
ubuntu-latest - python 3.8 - Style	39s
ubuntu-latest - python 3.8 - Docs	47s
Release	0s
	13m 55s

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