Use MessageAnnotation for span.log

This changes the behaviour of span.log slightly as to be seen in #103

If you want the same behaviour as before you need to migrate your span.log usage like this:

// before 3.0

cy.log({foo: "bar", one: "two"})

// after 3.0

cy.setTag("foo", "bar");
cy.setTag("one", "two");

Thank you @ashishmahi for sending in the PR 💯