Add field `TaskContext.preemptible` #964

Job	Run time
Docker	20s
Pydantic V1	45s
Compatibility	38s
ubuntu-latest - python 3.8 - Lint	46s
ubuntu-latest - python 3.8 - Type check	41s
ubuntu-latest - python 3.8 - Unit tests	1m 48s
ubuntu-latest - python 3.8 - Doc tests	2m 1s
ubuntu-latest - python 3.8 - Doc tests (B)	1m 47s
ubuntu-latest - python 3.8 - Images	1m 38s
ubuntu-latest - python 3.8 - Jobs	37s
ubuntu-latest - python 3.8 - Experiments	44s
ubuntu-latest - python 3.8 - Datasets	1m 7s
ubuntu-latest - python 3.8 - Sweep example	1m 36s
ubuntu-latest - python 3.8 - Build	35s
ubuntu-latest - python 3.8 - Style	36s
ubuntu-latest - python 3.8 - Docs	47s
Release	0s
	16m 26s

Provide feedback