refactor(blobmanager/s3accesspoint): slim AssumeRole session policy (… #402

Job	Run time
Release Gate	20s
test / Test (main-module)	5m 16s
test / Test (artifact-cas)	1m 29s
test / Test (cli)	3m 19s
test / Test (controlplane)	8m 3s
Release CLI and control-plane/artifact-cas container images	0s
Release Gate	10s
test / Test (cli)	3m 19s
test / Test (artifact-cas)	1m 29s
test / Test (main-module)	5m 16s
test / Test (controlplane)	8m 3s
Release CLI and control-plane/artifact-cas container images	17m 7s
	53m 51s

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