hcs-lib: messageFromApp - with encryption

Add the following methods to the builder

.setKey(key) - so the library can encrypt or sign if necessary
.withEncryption(boolean) - so the app can override library default per message
.withSignature(boolean) - so the app can override library default per message

If library has defaults set (through config) for signing, encrypting then encrypt and sign before sending unless overridden for this specific message.

Note: the format of the message sent to HCS needs to indicate whether it's signed, encrypted, this will either require specific message formatting POJO->Serialisation, or a message header containing the requisite flags and field sizes if necessary.