Rename `OpenChannelRequest::is_public` to `is_announced`

Rename `OpenChannelRequest::is_public` to `is_announced`

Referring to announced/unannounced channels as 'public'/'private'
regularly leads to confusion on what they are and when which should be
used. To avoid perpetuating this confusion, we should avoid referring to
announced channels as 'public' in our API.

Here we rename the recently introduced field in `OpenChannelRequest`
(which doesn't break released API), and will align the pre-existing
instances of `is_public` in the following commit (which will break API).