refactor: Refactoring EC2InstanceWorker to split out PosixInstanceWorker and WindowsInstanceWorker

[ttblanchard]

What was the problem/requirement? (What/Why)

When we added the ability to support Windows workers we didn't use proper abstractions and ended up with a whole lot of code that does checks like this:

        if self.configuration.operating_system.name == "AL2023":
            userdata = self.linux_userdata(s3_files)
        else:
            userdata = self.windows_userdata(s3_files)

This is hard to maintain, read, and extend.

There were also some minor bugs discovered in the windows worker fixture implementation that were fixed as part of this change.

What was the solution? (How)

Make EC2InstanceWorker an abstract class with two concrete classes PosixInstanceWorker and WindowsInstanceWorker which inherit from the EC2InstanceWorker allowing us to remove all the branching code.

What is the impact of this change?

More readable and maintainable test fixtures.

How was this change tested?

I tested with a custom AMI with the worker agent installed - submitting jobs using both windows and linux workers.

Was this change documented?

No

Is this a breaking change?

Yes - libraries that use this package will need to update their tests to explicitly use either PosixInstanceWorker or WindowsInstanceWorker.

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jusiskin]

This is reported as a breaking change in the PR description but does not indicate so in the PR/commit title according to conventional commits.

I recommend using the title and footer style. This has the benefits of both:

1. identifying breaking changes when using git log or looking at commit history in GitHub
2. providing a clearer description of the breaking change in the footer which can be used to automatically populate the changelog on release

Changes have been addressed and others have followed up with reviews of their own.