Add support for GP3 volume type

#### Description

Consider making it the default volume type if it's cheaper.

Also take into consideration making the speed and IOPS configurable like in https://cloudwiry.com/ebs-gp3-vs-gp2-pricing-comparison/.

Requires eksctl >= 0.37

#### Additional context

https://github.com/cortexlabs/cortex/pull/1842