manifest_xml: initial support for <contactinfo>

It will be used to let manifest authors self-register contact info.
This element can be repeated, and any later entries will clobber
earlier ones. This would allow manifest authors who extend
manifests to specify their own contact info.

It would have 1 required attribute: bugurl.
"bugurl" specifies the URL to file a bug against the manifest owner.

<contactinfo bugurl="bug-url"/>

TODO: This CL only implements the parsing logic and further work
will be in followup CLs.

Tested the code with the following commands.

$ ./run_tests tests/test_manifest_xml.py
$ ./run_tests -v

Bug: [google internal] b/186220520.
Change-Id: I47e765ba2dab5cdf850191129f4d4cd6b803f451
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305203


Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>