Enchance /crawl API endpoint with local server information (RIPD-1644):

The /crawl API endpoint allows developers to examine the structure of
the XRP Ledger's overlay network.

This commit adds additional information about the local server to the
/crawl endpoint, making it possible for developers to create data-rich
network-wide status dashboards.

Related:
 - https://developers.ripple.com/peer-protocol.html
 - https://github.com/ripple/rippled-network-crawler

cfg/rippled-example.cfg

@@ -20,7 +20,9 @@
 #
 #   7. Voting
 #
-#   8. Example Settings
+#   8. Misc Settings
+#
+#   9. Example Settings
 #
 #-------------------------------------------------------------------------------
 #
@@ -1039,6 +1041,46 @@
 #   [signing_support]
 #   true
 #
+# [crawl]
+#
+#   List of options to control what data is reported through the /crawl endpoint
+#   See https://developers.ripple.com/peer-protocol.html#peer-crawler
+#
+#   <flag>
+#
+#       Enable or disable access to /crawl requests. Default is '1'
+#
+#   overlay = <flag>
+#
+#       Report information about peers this server is connected to, similar
+#       to the "peers" RPC API. Default is '1'.
+#
+#   server = <flag>
+#
+#       Report information about the local server, similar to the "server_state"
+#       RPC API. Default is '1'.
+#
+#   counts = <flag>
+#
+#       Report information about the local server health counters, similar to
+#       the "get_counts" RPC API. Default is '0'.
+#
+#   unl = <flag>
+#
+#       Report information about the local server's validator lists, similar to
+#       the "validators" and "validator_list_sites" RPC APIs. Default is '1'.
+#
+#   Example:
+#
+#   [crawl]
+#   0
+#
+#   [crawl]
+#   overlay = 1   # report peer overlay info
+#   server = 1    # report local server info
+#   counts = 0    # do not report server counts
+#   unl = 1       # report server validator lists
+#
 #-------------------------------------------------------------------------------
 #
 # 9. Example Settings