[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`.

script/check-scan-build

@@ -69,6 +69,7 @@ do_scan_build()
         "-DOPENTHREAD_CONFIG_MLE_STEERING_DATA_SET_OOB_ENABLE=1"
         "-DOPENTHREAD_CONFIG_MPL_DYNAMIC_INTERVAL_ENABLE"
         "-DOPENTHREAD_CONFIG_NEIGHBOR_DISCOVERY_AGENT=1"
+        "-DOPENTHREAD_CONFIG_NETDATA_PUBLISHER_ENABLE=1"
         "-DOPENTHREAD_CONFIG_PING_SENDER_ENABLE=1"
         "-DOPENTHREAD_CONFIG_PLATFORM_FLASH_API_ENABLE=1"
         "-DOPENTHREAD_CONFIG_PLATFORM_RADIO_COEX_ENABLE=1"