Add a new database API to get signed block header

[abitmore]

User Story

As a client app developer I want a database API which returns the signed block header of a given block number (or perhaps multiple blocks) so that I can calculate the block ID without getting the entire block.

Additional Context (optional)

We have a get_block_header API but it does not return the witness signature thus unable to calculate the block ID with the result. The get_block API is usable, but it returns too much data.

Impacts
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

• 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 Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation

[abitmore]

Perhaps update the existing get_block_header API to return the signature too?

By the way, the get_block_header_batch API is similiar.

[abitmore]

Done via #2641.

[abitmore]

Optimizing via #2652.

[abitmore]

Done via #2641 and #2652.