Doc/Bug: `/_chamber/store-config` requires `ssm:GetParameters` permission in IAM policies

[kopach]

Chamber reads the SSM parameter /_chamber/store-config on write

1. Create an IAM policy that scopes ssm:GetParameter* to a specific service path:

{
  "Statement": [
    {
      "Effect": "Allow",
      "Action": ["ssm:GetParameter*"],
      "Resource": "arn:aws:ssm:eu-west-1:ACCOUNT_ID:parameter/my-service/*"
    }
  ]
}

2. run

chamber write my-service TEST_KEY "hello"

Expected Behavior

The command succeeds, or the documentation explicitly lists /_chamber/store-config as a required accessible path.

Actual Behavior

Error: operation error SSM: GetParameters, https response error StatusCode: 400,
api error AccessDeniedException: User: arn:aws:iam::ACCOUNT_ID:user/USERNAME is not
authorized to perform: ssm:GetParameters on resource:
arn:aws:ssm:eu-west-1:ACCOUNT_ID:parameter/_chamber/store-config because no
identity-based policy allows the ssm:GetParameters action