[api] add API to extract an extended address from an IPv6 address (#11933)

The application of the P2P module may only record the P2P peer's
link-local address for communication. This commit adds an API to
convert the peer's link-local unicast address to the peer's extended
address for tearing down the P2P link.

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (545)
+#define OPENTHREAD_API_VERSION (546)
 
 /**
  * @addtogroup api-instance

include/openthread/ip6.h

@@ -576,6 +576,14 @@ bool otIp6IsLinkLocalUnicast(const otIp6Address *aAddress);
  */
 void otIp6FormLinkLocalAddressFromExtAddress(const otExtAddress *aExtAddress, otIp6Address *aAddress);
 
+/**
+ * Extracts the MAC Extended Address from the Interface Identifier of the given IPv6 address.
+ *
+ * @param[in]  aAddress     A pointer to the IPv6 address.
+ * @param[out] aExtAddress  A pointer to output the MAC Extended Address (generated from the IID).
+ */
+void otIp6ExtractExtAddressFromIp6AddressIid(const otIp6Address *aAddress, otExtAddress *aExtAddress);
+
 /**
  * Test if two IPv6 prefixes are the same.
  *

src/core/api/ip6_api.cpp

@@ -178,6 +178,11 @@ void otIp6FormLinkLocalAddressFromExtAddress(const otExtAddress *aExtAddress, ot
     AsCoreType(aAddress).SetToLinkLocalAddress(AsCoreType(aExtAddress));
 }
 
+void otIp6ExtractExtAddressFromIp6AddressIid(const otIp6Address *aAddress, otExtAddress *aExtAddress)
+{
+    AsCoreType(aExtAddress).SetFromIid(AsCoreType(aAddress).GetIid());
+}
+
 bool otIp6ArePrefixesEqual(const otIp6Prefix *aFirst, const otIp6Prefix *aSecond)
 {
     return AsCoreType(aFirst) == AsCoreType(aSecond);