feat: BN014 detect duplicate policies

[DinoChiesa]

I don't know why, but previously the EP001 plugin, which is intended to check for CORS policy placement, also made a lame attempt to check for duplicate CORS policies. That always failed, because the check was naive: any policy that had a different name was considered to be different. Therefore it never detected a duplicate.

So that code needed to go. But it seems like it's possible that someone might have multiple differently named policies (CORS or otherwise), with identical configuration, in the same proxy bundle. And if that might occur, it would be good to check for that and issue a warning. So I implemented that check in a new plugin, BN014. It has "pretty good" XML policy comparison logic: two different policies will not be judged to be different if the only differences are in whitespace or indenting. They will be judged to be different if the element order is different.

Not sure how useful this will be. But rather than remove the duplicate policy check, I thought it would be better to keep it, as a new plugin.

This also increases code coverage for the EP001 plugin.