Helm: add eni option to use iam-role

[bluestealth]

Prevents helm from adding secret env in particular AWS_DEFAULT_REGION for AWS pod identity if using an iam-role.

It seems that this prevents the eks-pod-identity-webhook from adding the entry if missing.

level=warning msg="Unable to synchronize EC2 VPC list" error="operation error EC2: DescribeVpcs, failed to sign request: failed to retrieve credentials: failed to retrieve credentials, operation error STS: AssumeRoleWithWebIdentity, failed to resolve service endpoint, an AWS region is required, but was not found" subsys=eni
level=fatal msg="Unable to start eni allocator" error="Initial synchronization with instances API failed" subsys=cilium-operator-aws

Fixes: #13270

Add iamRole option to eni in Helm chart values to allow using serviceaccounts for iam roles on cilium-operator

[qmonnet]

Looks good, although my knowledge of AWS is too limited to provide a correct review on the content.

[christarazi]

LGTM, I think the second commit can be squashed with the first

[bluestealth]

@christarazi done

[kkourt]

Same as @qmonnet

[qmonnet]

test-me-please
I see Chris is part of the AWS team and approved, so I removed the review request for Thomas.

[qmonnet]

test-1.19-4.19

[qmonnet]

test-1.20-4.9