HBAR transfer block

### Problem description
There is currently no easy way to pay validators and verifiers (or any other policy participants) for their services directly from within a Guardian policy. Adding a simple HBAR transfer block could be highly valuable, because:
(a) Currently policy participants have to handle payments for services outside of the policy, which is a hassle - you have to use two platforms (the Guardian + some payment system) to perform one task (e.g., getting a report reviewed). Having an HBAR transfer block will allow you to handle both aspects of a task from within the Guardian.
(b) One of the big problems with institutionalised/traditional VVBs is that they are very expensive and usually require large up-front payments. Having an HBAR transfer block available to policies will make micropayments (or "pay-as-you-go" payments) possible, opening up the way for the develpment of validation and verification paradigms that accommodate project developers who cannot afford traditional VVBs but who still want to produce high-quality credits.

### Requirements
At least the following scenario should be possible:
The HBAR transfer block is preceded by a requestVcDocumentBlock. The HBAR transfer block uses the fields in the received VC to determine how much HBAR to transfer and to whom, e.g., send `document.credentialSubject.0.field0` HBAR to the account specified by `document.credentialSubject.0.field1`. If the recipient field is left unspecified during policy creation, the block should default to the issuer of the VC document.

### Definition of done
An HBAR transfer block has been added to the list of available policy blocks. The HBAR transfer block works for the scenario presented under 'Requirements' above.