Update getblockchaininfo RPC method

[mpguerra]

Motivation

In order to enable zcashd deprecation, we need to ensure that RPC methods that are used by existing zcashd consumers are implemented in zebra to cater to their usecases.

Specification

zcashd's getblockchaininfo RPC method is documented here: https://zcash.github.io/rpc/getblockchaininfo.html

Zebra already partially implements the getblockchaininfo RPC method, however, it is missing the following fields:

• "initial_block_download_complete": xx, (boolean) true if the initial download of the blockchain is complete
• "headers": xxxxxx, (numeric) the current number of headers we have validated
• "difficulty": xxxxxx, (numeric) the current difficulty
• "verificationprogress": xxxx, (numeric) estimate of verification progress [0..1]
• "chainwork": "xxxx" (string) total amount of work in active chain, in hexadecimal
• "size_on_disk": xxxxxx, (numeric) the estimated size of the block and undo files on disk
• "commitments": xxxxxx, (numeric) the current number of note commitments in the commitment tree
• "chainSupply": (object) information about the total supply
  • "monitored": xx, (boolean) true if the total supply is being monitored
  • "chainValue": xxxxxx, (numeric, optional) total chain supply after this block, in ZEC
  • "chainValueZat": xxxxxx, (numeric, optional) total chain supply after this block, in zatoshis
• For the valuePools object:
  • "monitored": xx, (boolean) true if the pool is being monitored
• "softforks": (array) status of softforks in progress
  • "id": "xxxx", (string) name of softfork
  • "version": xx, (numeric) block version
  • "enforce": (object) progress toward enforcing the softfork rules for new-version blocks
    • "status": xx, (boolean) true if threshold reached
    • "found": xx, (numeric) number of blocks with the new version found
    • "required": xx, (numeric) number of blocks required to trigger
    • "window": xx, (numeric) maximum size of examined window of recent blocks
  • "reject": { ... } (object) progress toward rejecting pre-softfork blocks (same fields as "enforce")

We need to:

1. Check which fields, if any, are needed by existing zcashd consumers.
2. Implement the required changes to support the relevant usecases.

Related work

We might want to do #8939 at the same time