feat: add support for custom sized root block devices.

[ethanholz]

This adds new support for new custom root devices when provisioning AWS instances. It adds the ability to read and modify the existing block devices for the AMI, however this requires adding the ec2:DescribeImages permission. This change will not work if you do not edit the permissions of the IAM policy to be aligned with this:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:RunInstances",
        "ec2:TerminateInstances",
        "ec2:DescribeInstances",
        "ec2:DescribeInstanceStatus",
        "ec2:DescribeImages"
      ],
      "Resource": "*"
    }
  ]
}

Unit testing is not written for this but a brief integration test can be found using this branch here: https://github.com/omsf-eco-infra/openmm-gpu-test/actions/runs/12698008379. I think proper testing can be added for this when we migrate to start-aws-gha-runner, since most of this testing is about argument validation.

[dwhswenson]

In addition to the tiny documentation update, would it be possible to add a small test to cover this functionality?

[dwhswenson]

Add: "If not given, defaults to ?? GB" (fill in correct value for ?? -- is that 30? can't remember)

[ethanholz]

This is dependent on the AMI. If you don't provide us with one, we default to the AMI.

[ethanholz]

I can improve the docs to reflect that though.

[dwhswenson]

Fair enough. Probably good to point that out, though.

[ethanholz]

It is 30 for most images, but the Deep Learning ones use 75 GB root devices.

[dwhswenson]

Ah, missed the mention of testing in the PR message (jumped straight to the code). Just add the docs improvement and this should be good to go.

[codecov]

Codecov Report

Attention: Patch coverage is 29.16667% with 17 lines in your changes missing coverage. Please review.

Project coverage is 90.78%. Comparing base (cc804d9) to head (cf84d64).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gha_runner/clouddeployment.py	23.80%	16 Missing ⚠️
src/gha_runner/__main__.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   95.20%   90.78%   -4.43%     
==========================================
  Files           3        3              
  Lines         334      358      +24     
==========================================
+ Hits          318      325       +7     
- Misses         16       33      +17     

Flag	Coverage Δ
unittests	90.78% <29.16%> (-4.43%)	⬇️

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