Investigate if we need to have global STS endpoint

[jaydeokar]

What would you like to be enhanced:
Investigate if we need to have a global fallback sts endpoint as per this comment

amazon-vpc-resource-controller-k8s/pkg/aws/ec2/api/wrapper.go

Lines 536 to 539 in dd3564c

	// Get the global sts end point
	// TODO: we should revisit the global sts endpoint and check if we should remove global endpoint
	// we are not using it since the concern on availability and performance
	// https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_enable-regions.html

Also this piece of code where if regional STS !=global STS we still end up using the regional STS endpoint. It should be global STS if regional is not available.

amazon-vpc-resource-controller-k8s/pkg/aws/ec2/api/wrapper.go

Lines 547 to 549 in dd3564c

	if regionalSTSEndpoint.URL != globalSTSEndpoint.URL {
	globalProvider := &stscreds.AssumeRoleProvider{
	Client: e.createSTSClient(userStsSession, client, regionalSTSEndpoint, sourceAcct, sourceArn),

Why is the change needed and what use case will it solve: