Adds a convenient way of iterating PubSub messages

[anarthal]

Adds push_view and push_parser. This is an input view range that allows parsing PubSub messages from a node tree.
Adds a doc page on how to use server pushes.
Adds reference docs to set_receive_response.

close #349

[mzimbres]

My only concern with this PR is that this push parser skips non-push messages silently. This might make it impossible to understand user issues. As we iterate over the messages, would it be possible to the iterator to push a reference to the skipped nodes into the push_parser. then we could at least log them after the loop if necessary

      auto parser = push_parser(resp.value());
      for (push_view elem : parser) {
           ...
      }
      for (auto node : parser.get_skipped()) {
           ...
      }

We could provide a macro to simplify that logging.

[mzimbres]

Good job, I think we should not delay this anymore as it is very hard to process push without that.

[anarthal]

Hm, storing references would require allocating. I could add an extended API to the parser (or even create a similar push_parser_ext) that also yields skipped messages. For instance:

struct push_view { /* as is today */ };
using push_or_skipped = variant<
   push_view, // a valid push
   span<const resp3::node_view> // skipped message
>;
class push_parser_ext {
   // value type is push_or_skipped
};

I don't think we can make it to this release though (needs to be on master for this Wednesday). So if you're okay with it, I'll merge it as-is, and think of adding this for next release.

I also hope to implement #387 so we can completely get rid of errors in the push buffer, making this issue less important.