add validator script

[oliviassss]

Issue #, if available:

Description of changes:
Add a validator script to check

1. aws-node vpc cni version is >= 1.19.4
2. aws-node container env vars
3. VPC subnets have /12 CIDR with prefix reservation

Tested in dev cluster

% ./eks_cluster_validator.sh 
Checking aws-node VPC CNI version...
[PASS] aws-node VPC CNI version 1.19.5 is >= 1.19.4
Checking aws-node container environment variables...
[FAIL] DISABLE_LEAKED_ENI_CLEANUP is not set to 'true'. Current value: 
[PASS] ENABLE_PREFIX_DELEGATION is set to 'true'
[PASS] MINIMUM_IP_TARGET is set to '30'
[PASS] WARM_IP_TARGET is set to '5'
Checking VPC subnets for /12 CIDR blocks and prefix delegation...
[WARN] Subnet subnet-0f358dc6e17a750e5 has /18 CIDR block (not /12)
[WARN] Subnet subnet-0de5770ba205ed3cf has /18 CIDR block (not /12)
[WARN] Subnet subnet-0cd7407b3ff938260 has /18 CIDR block (not /12)
[FAIL] No subnets with /12 CIDR blocks and prefix delegation found
Validation FAILED. Please address the issues highlighted above.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.