security/access: Add an "access tag" caveat.

This commit introduces a Caveat implementation that can be used to
restrict methods invokable by a principal based on the tags on the
method being invoked.

My first intented use of this is for the "debug browse" tool,
wherein one user can delegate debugging of their server to
another - restricting the delegate so that it can only invoke
methods with the Debug or Resolve label - and thus not mess
with state of the server being debugged.

MultiPart: 1/2

Change-Id: I7605c230792ac4b0ecd0e42e54becc286b2af9d1
6 files changed