Allow signing Omni transactions with the keys stored in secure hardware

[msgilligan]

The RivetzSDK allows signing of Bitcoin transactions with a private key stored…

… in the Trusted Execution Environment (TEE) available in many modern devices. The TEE is a hardware environment that runs small applets outside the main OS. This protects sensitive code and data from malware or snooping with purpose-built hardware governed by an ecosystem of endorsements, beginning with the device manufacturer.

Currently, the method OmniTxBuilder.createSignedOmniTransaction() requires local ECKey objects with private keys and the method OmniTxBuilder.createOmniTransaction() creates unsigned, incomplete transactions (no change output.) To support Rivetz we'll need some form of pluggable signing.

[mawenpeng]

I also need this feature. I'm trying to find a workaround.