[netdata] adding NetworkData::Publisher (#6768)

This commit implements a new feature "Network Data Publisher" which
provides mechanisms to limit the number of similar entries (service
and/or prefix) in the Thread Network Data by monitoring the Network
Data and managing if or when to add or remove entries. This feature is
enabled using `OPENTHREAD_CONFIG_NETDATA_PUBLISHER_ENABLE` config, or
`NETDATA_PUBLISHER` in autoconf, or `OT_NETDATA_PUBLISHER` cmake
option.

This commit adds support for publishing DNS/SRP anycast/unicast
service, on-mesh prefix, and external route prefix entries.

When there is a request to publish an entry, the `Publisher` monitors
the Network Data and counts the number of similar entries. If there
are fewer entries than a desired target number, the entry is added
after a short random delay.

If there are too many similar entries, `Publisher` starts the process
of removing its own entry (again after some random wait time). When
removing entries, certain entries are preferred over others (e.g., an
entry from a router over one from an end-device or if they are from
the same type of node, the one with smaller RLOC16). If `Publisher`
determines that its own entry is a preferred one, it adds an extra
wait time before removing its entry. This gives higher chance for a
non-preferred entry from another device to be removed before removing
a preferred entry which helps towards quicker convergence of the
process to the desired number of entries.

On-mesh prefix and external route entries have a "preference" field.
When publishing such an entry, a matching entry in the network data is
counted only if its preference is same or higher than the entry's
preference. This ensures that a device with a higher preference entry
publishes its entry even when there are many lower preference similar
entries in the network data (potentially causing a lower preference
entry to be removed).

This commit also adds `test_netdata_publisher.py` to verify the
behavior of the `Publisher`.

tests/scripts/thread-cert/node.py

@@ -1715,6 +1715,34 @@ class NodeImpl:
         self.send_command('netdata register')
         self._expect_done()
 
+    def netdata_publish_dnssrp_anycast(self, seqnum):
+        self.send_command(f'netdata publish dnssrp anycast {seqnum}')
+        self._expect_done()
+
+    def netdata_publish_dnssrp_unicast(self, address, port):
+        self.send_command(f'netdata publish dnssrp unicast {address} {port}')
+        self._expect_done()
+
+    def netdata_publish_dnssrp_unicast_mleid(self, port):
+        self.send_command(f'netdata publish dnssrp unicast {port}')
+        self._expect_done()
+
+    def netdata_unpublish_dnssrp(self):
+        self.send_command('netdata unpublish dnssrp')
+        self._expect_done()
+
+    def netdata_publish_prefix(self, prefix, flags='paosr', prf='med'):
+        self.send_command(f'netdata publish prefix {prefix} {flags} {prf}')
+        self._expect_done()
+
+    def netdata_publish_route(self, prefix, flags='s', prf='med'):
+        self.send_command(f'netdata publish route {prefix} {flags} {prf}')
+        self._expect_done()
+
+    def netdata_unpublish_prefix(self, prefix):
+        self.send_command(f'netdata unpublish {prefix}')
+        self._expect_done()
+
     def send_network_diag_get(self, addr, tlv_types):
         self.send_command('networkdiagnostic get %s %s' % (addr, ' '.join([str(t.value) for t in tlv_types])))