[thread-tlvs] define min/max num of addrs under Ip6AddressesTlv (#7834)

This commit moves the definitions of constants related to min/max
number of IPv6 addresses that can be included in an IP6 Addresses TLV
to `Ip6AddressesTlv` class. It also adds a public OT constant in
`ip6.h` related to this for MLR feature which is then used in `cli`
and `ncp` modules so to avoid the use of OT core internal constant
from CLI/NCP.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (220)
+#define OPENTHREAD_API_VERSION (221)
 
 /**
  * @addtogroup api-instance

include/openthread/ip6.h

@@ -781,6 +781,8 @@ typedef void (*otIp6RegisterMulticastListenersCallback)(void *              aCon
                                                         const otIp6Address *aFailedAddresses,
                                                         uint8_t             aFailedAddressNum);
 
+#define OT_IP6_MAX_MLR_ADDRESSES 15 ///< Max number of IPv6 addresses supported by Multicast Listener Registration.
+
 /**
  * This function registers Multicast Listeners to Primary Backbone Router.
  *