Store account custom data in ElasticSearch database

**User Story**

As a user, I would like account custom data (currently handled with the `custom_operations` plugin) to also be optionally stored in an ElasticSearch database (like the `es_objects` plugin) for more flexibility in querying.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. 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.)
- [x] UX (the User Experience)
- [ ] Other (please add below)

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

Start by reading the existing custom_operations and es_objects plugins to understand how account custom data and Elasticsearch-backed objects are handled. Then clarify the optional-storage configuration, query behavior, and test cases with the maintainers; done means the requirements are defined, implemented, and verified for both enabled and disabled storage.

Written by the indexing model from the issue text.