Proposal: show DP information in the Admin API

[Zexuan Luo <sp...@apache.org>]

Background

We sometimes want to get the current number of DPs and specific
information about each DP on the Admin API.
Currently, we have the server-info plugin that can report the
information of that DP on the DP.

Solution

Move server-info from a separate plugin into the main code.
To reduce the impact on users who have server-info explicitly enabled,
we keep server-info as a plugin, but it's just an empty shell that
prints a Warning log.

In the Admin API, we add the GET /apisix/admin/data_planes interface
to support getting information about a single data_plane or all
data_planes.

In documentation, we add a new document "DP Information Reporting",
which describes how DP reports information.