qapi/introspect.py: create a typed 'Annotated' data strutcure

qapi/introspect.py: create a typed 'Annotated' data strutcure

Presently, we use a tuple to attach a dict containing annotations
(comments and compile-time conditionals) to a tree node. This is
undesirable because dicts are difficult to strongly type; promoting it
to a real class allows us to name the values and types of the
annotations we are expecting.

In terms of typing, the Annotated<T> type serves as a generic container
where the annotated node's type is preserved, allowing for greater
specificity than we'd be able to provide without a generic.

Signed-off-by: John Snow <[email protected]>
Message-Id: <20210216021809[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>