THRIFT-564. Support arbitrary annotations on struct fields

This is subtly different from a type annotation, since some bits of
metadata only make sense in the context of a single structure field,
like whether the field is required, or whether the C++ code should use
a pointer for it.

This change doesn't define any meaningful annotations.  It just sets up
the parsing infrastructure.

I have no idea if $10 will work with older versions of yacc.  It seems
to work fine with bison.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@919325 13f79535-47bb-0310-9956-ffa450edef68