(de) Pagination support

[zacps]

I would like to be able to do something like:

client = httpx.Client(pagination=httpx.CursorPagination(cursor="next", results="data"))
client.get("api:80/myRoute")

and with responses:

{
    "next": "api:80/myRoute?page=1",
    "data": [
        {...},
        {....}
    ]
}

return the data items concatenated.

Similarly this could also support pagination via query parameters directly:

client = httpx.Client(pagination=httpx.QueryPagination(param="page", results="data", start=0, stop_on=httpx.QUERY_PAGINATION_404))

Of course, this requires httpx to read and understand the response format. This can of course be done with the content-type if it's specified, and it would be simple enough to only support this for JSON, but it would be nice to be able to support messagepack and alternatives as well, at which point httpx would need to have pluggable parsing implementations.