Created a new label for issues in p4lang/p4runtime repo

[Andy Fingerhut]

My apologies if you see a flurry of emails today in your inbox.

I created a new label p4-language-compatibility for issues in the p4lang/p4runtime repo, and labeled all open issues I know about that are relevant to that.

There is a new issue related to compatibility with changes to version 1.2.4 of the language spec, since version 1.2.3 was released, because I just realized earlier today that I haven't actually checked through those yet.  I should have an initial list of new issues to discuss on that topic created in the next day or two.

Andy