otBorderAgentEphemeralKeyCallback

Callback function pointer to signal changes related to the Border Agent's ephemeral key. This callback is invoked whenever: - The Border Agent starts using an ephemeral key. - Any parameter related to the ephemeral key, such as the port number, changes. - The Border Agent stops using the ephemeral key due to: - A direct call to `otBorderAgentClearEphemeralKey()`. - The ephemeral key timing out. - An external commissioner successfully using the key to connect and then disconnecting. - Reaching the maximum number of allowed failed connection attempts. Any OpenThread API, including `otBorderAgent` APIs, can be safely called from this callback.