[BuildImage] Support build-image with UpdateOs=Enabled in proxied environments

[gmarciani]

Description of changes

Support build-image with UpdateOs enabled in proxied environments.

With this fix we now execute the proxy configuration in the ImageBuilder component update_and_reboot.yaml.
The proxy config steps mimic what we already do in the dedicated cookbook recipe.

Alternative approaches

As an alternative approach we could invoke the cookbook recipe to setup the proxy. In this way we would have the logic to configure the proxy in a single place. However, I am not doing it here because this would break the following two rules we have always complied with:

1. update&reboot component must not depend on any cookbook artifact. This rule can be revised b/c
2. we do not call internal recipes. this is a weak argument b/c we could anyways create a new entrypoint just to configure the proxy.

Tests

ONGOING

test-suites:
  createami:
    test_createami.py::test_build_image_no_internet:
      dimensions:
        # RHEL family (rhel8, rhel9) -> Red Hat Enterprise Linux reservation platform
        - regions: [{{ g4dn_2xlarge_CAPACITY_RESERVATION_2_INSTANCES_3_HOURS_NOPG_rhel9 }}]
          instances: ["g4dn.2xlarge"]
          oss: ["rhel8", "rhel9"]
        # Non-RHEL family (alinux2023, ubuntu2204, ubuntu2404, rocky8, rocky9) -> Linux/UNIX reservation platform
        - regions: [{{ g4dn_2xlarge_CAPACITY_RESERVATION_5_INSTANCES_3_HOURS_NOPG_ubuntu2404 }}]
          instances: ["g4dn.2xlarge"]
          oss: ["alinux2023", "ubuntu2204", "ubuntu2404", "rocky8", "rocky9"]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.