ot::MeshCoP::BorderAgent::SetEphemeralKey() method

Sets the ephemeral key for a given timeout duration. The ephemeral key can be set when the Border Agent is already running and is not currently connected to any external commissioner (i.e., it is in `kStateStarted` state). To terminate active commissioner sessions, use the `Disconnect()` function. The given @p aKeyString is directly used as the ephemeral PSK (excluding the trailing null `\0` character). Its length must be between `kMinEphemeralKeyLength` and `kMaxEphemeralKeyLength`, inclusive. Setting the ephemeral key again before a previously set one is timed out will replace the previous one and will reset the timeout. During the timeout interval, the ephemeral key can be used only once by an external commissioner to establish a connection. After the commissioner disconnects, the ephemeral key is cleared, and the Border Agent reverts to using PSKc. If the timeout expires while a commissioner is still connected, the session will be terminated, and the Border Agent will cease using the ephemeral key and revert to PSKc.