Feature: Enable runner instance volume type choices

[ryancormack]

Summary

Currently ec2 instances are launched with a gp2 volume type by default, however, gp3 volume types can be more performant and cheaper. The host runner can be launched with gp3 EBS volume types, but it's not possible to enable gp3 volume types. The GitLab Docker EC2 Driver supports setting this value, https://gitlab.com/gitlab-org/ci-cd/docker-machine/-/blob/main/drivers/amazonec2/amazonec2.go#L211

Suggested Implementation

I propose to allow setting a new variable/option of runners_volume_type, which will default to gp2 to maintain backwards compatibility, that can then be passed into the Runners Machine Config in the launch template

[ryancormack]

I'm happy to pick up the work on this issue