feat(eks): cross-account support

[Gearheads]

What this PR does / why we need it:
This PR will allow EKS users to leverage cross-account support. This means the feature will allow users to provide a new flag called --assume-role-arn, kconnect will login will the main credentials, and then switch to a different AWS IAM role within another AWS Account.

This is helpful for users who want to be part of a single AD group in a central AWS account, which then assumes and gives access to any EKS cluster instead of having to be in several different AD groups for each AWS account.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Testing:

• make ci completed successfully
• was able to connect to a cluster, and assume the new AWS IAM role

% ./kconnect use eks --namespace <namespace> --region us-east-1 --username *** --password *** --assume-role-arn arn:aws:iam::<AWS-account>:role/<AWS-IAM-role>
info    kconnect - the Kubernetes Connection Manager CLI        {"version": ""}
info    authenticating user     {"app": "kconnect", "provider": "saml"}
? Select AWS role Account: <AWS-account-name> (<AWS-account>) / <AWS-IAM-role>
info    requesting AWS credentials using SAML   {"provider": "saml", "sp": "aws"}
info    discovering clusters    {"app": "kconnect", "provider": "eks"}
info    discovering EKS clusters        {"app": "kconnect", "provider": "eks"}
? Select a cluster <cluster-name>
? Do you want to set an alias? No
info    Command to reconnect using this alias: kconnect to      {"app": "kconnect"}
info    setting current context {"context": "kconnect@<eks-cluster-name>"}
info    kubeconfig updated      {"path": "/path/to/KUBECONFIG"}

% ./kconnect ls | head -n 3                                                                                                                                                     
info    kconnect - the Kubernetes Connection Manager CLI        {"version": ""}
CUR   ID                           ALIAS    PROVIDER   PROVIDERID               IDENTITY   USER     TIME LEFT
>     01hffm07j9m8ck60t3w3cjr5zy            eks        arn:aws:eks:<provider>   saml       <user>   59m53s

% kubectl get nodes
Error from server (Forbidden): nodes is forbidden: User "arn:aws:iam::<AWS-assumed-account>:role/<AWS-assumed-IAM-role>" cannot list resource "nodes" in API group "" at the cluster scope