aws-backend: add support for running instances on multiple subnets

[timbrown5]

Update the AWS backend (and instance config) to add the ability to run instance on a set of subnets (and
so availability zone).

The main use-case for this is to allow instance to come up in multiple availability zones. A user would create one (or more) subnets in an availability zone and the list the subnets in the subnet_configs section. This has two main advantages:

• There is some crude load balancing (subnets are picked at random from the list)
• If an instance fails to start we can retry in another subnet. This will help in cases where an AZ has been starved of all the instance and instance not longer start.

To support this (without breaking existing configs) I added a new subnet_configs section to the instance config. This is mutually exclusive to the existing subnet_id section.
If subnet_configs is set, then the backend will choose one of the subnets at random and try to bring up the instance there. If this fails. then another subnet will be tried, until one works. If none of the subnets work, an the exception will be re-raised.

[timbrown5]

Note: I use aws SSO profiles for auth, so needed to add a check for the AWS_PROFILE env var to be able to run the tests.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 85.11%. Comparing base (8969d71) to head (3cec72a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
runner_manager/backend/aws.py	12.19%	36 Missing ⚠️

❌ Your patch status has failed because the patch coverage (20.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #700      +/-   ##
==========================================
- Coverage   86.68%   85.11%   -1.58%     
==========================================
  Files          35       35              
  Lines        1510     1545      +35     
==========================================
+ Hits         1309     1315       +6     
- Misses        201      230      +29     

Flag	Coverage Δ
api	62.84% <20.00%> (-1.06%)	⬇️
unit	67.24% <20.00%> (-1.17%)	⬇️

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[tcarmet]

Hey @timbrown5 ! Thank you for the contribution!

Haven't got the time to review it properly yet, but will do soon, in the meantime, don't worry about codecov complaining, we don't execute those tests in the CI, but do check the linting issues reported (should be easily fixed with a trunk fmt if you have trunk installed).

Cheers!

[tcarmet]

just a small change request, and let's merge it.

Thanks again

[timbrown5]

@tcarmet, the linter is complaining about this line, but it's not a addition, just moved from the original code. Do you have a preferred way to fix it (I've silenced some warnings before, but I'm not 100% that that's the best plan).

[tcarmet]

Thanks for letting me know, I'm not sure how to fix it as well, but let's merge it as is, I think it's minor and it won't complain on the next lint checks.