ot::Crypto::Ecdsa::P256::KeyPair::GetDerBytes() method

Gets the pointer to start of the key-pair buffer in DER format. Gives non-const pointer to the buffer and is intended for populating the buffer and setting the key-pair (e.g., reading the key-pair from non-volatile settings). The buffer contains `kMaxDerSize` bytes. After populating the buffer, `SetDerLength()` can be used to set the the number of bytes written.