Update getinfo 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 getinfo RPC method is documented here: https://zcash.github.io/rpc/getinfo.html

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

• version: xxxxx, (numeric) the server version
• protocolversion: xxxxx, (numeric) the protocol version
• walletversion: xxxxx, (numeric, optional) the wallet version, if wallet functionality is enabled
• balance: xxxxxxx, (numeric, optional) the total Zcash balance of the wallet, if wallet functionality is enabled
• blocks: xxxxxx, (numeric) the current number of blocks processed in the server
• timeoffset: xxxxx, (numeric) the time offset (deprecated; always 0)
• connections: xxxxx, (numeric) the number of connections
• proxy: "host:port", (string, optional) the proxy used by the server
• difficulty: xxxxxx, (numeric) the current difficulty
• testnet: true|false, (boolean) if the server is using testnet or not
• keypoololdest: xxxxxx, (numeric, optional) the timestamp (seconds since GMT epoch) of the oldest pre-generated key in the key pool, if wallet functionality is enabled
• keypoolsize: xxxx, (numeric, optional) how many new keys are pre-generated
• unlocked_until: ttt, (numeric, optional) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked, if wallet functionality is available and the wallet is encrypted
• paytxfee: x.xxxx, (numeric) the transaction fee rate set in ZEC per 1000 bytes
• relayfee: x.xxxx, (numeric) minimum relay fee rate for transactions in ZEC per 1000 bytes
• errors: "..." (string) message describing the latest or highest-priority error
• errorstimestamp: "..." (string) timestamp associated with the latest or highest-priority error

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.

[oxarbitrage]

I think we can get rid of all the optional wallet fields as Zebra will not be ever (with the current plan) able to provide them. This fields are walletversion, balance, keypoololdest, keypoolsize and unlocked_until.
Additionally, the timeoffset is deprecated so it should be safe to just remove it in Zebra.

[oxarbitrage]

Reference zcashd output:

$ ./src/zcash-cli getinfo
{
  "version": 6010050,
  "build": "v6.1.0-a3435336b",
  "subversion": "/MagicBean:6.1.0/",
  "protocolversion": 170120,
  "walletversion": 60000,
  "balance": 0.00000000,
  "blocks": 4608,
  "timeoffset": 0,
  "connections": 1,
  "proxy": "",
  "difficulty": 145299.3882313141,
  "testnet": false,
  "keypoololdest": 1739448263,
  "keypoolsize": 101,
  "paytxfee": 0.00000000,
  "relayfee": 0.00000100,
  "errors": "",
  "errorstimestamp": 1739448460
}
$