Update get_*_signatures APIs to support custom active authorities

**Bug Description**

Mentioned in https://github.com/bitshares/bitshares-core/issues/2048:
> ... BSIP 40 allows transactions to be signed by additional accounts, keys, and authorities that have been authorized by the required party. For example, Alice could authorize Bob to transfer funds from her account. Therefore, Bob should be able to sign such a transaction by signing with his account's active key. ...
>
> ... We need to fix the `get_potential_signatures` and `get_required_signatures` APIs to return keys related to custom active authorities. ...

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [x] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation