netdog: Move to quick-xml for XML serialization

[zmrow]

Description of changes:

`serde_xml_rs` has a few bugs that we were able to work around, but the
inability to serialize a `Vec` is something we can't work around.  This
change moves the netdog code to `quick-xml` for serialization.

This change also allows us to get rid of the _f: () struct member that worked around a serde_xml_rs bug. However, quick-xml is slightly more wordy in that you need to supply $unflatten attributes for anything that is a simple type and needs to have it's own tag; otherwise it gets made an attribute of the tag itself.

Testing done:

• All unit tests still pass
• Built and ran an aws-k8s-1.21 node successfully.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

^ Rebases on develop and resolves merge conflicts

[etungsten]

🍔

[stmcginnis]

Tests are happy and the code changes look good to me. Not super familiar with the code and any side effects changes might have though, so will leave to someone else to merge.