feat(rpc): populate some getblocktemplate RPC block header fields using the state best chain tip

[oxarbitrage]

Motivation

[More to be added]

Populate the following fields:

• capabilities
• target
• min time
• cur time
• bits
• height
• previous block hash

#5455

Solution

[TBA]

Review

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
• How do you know it works? Does it have tests?

Follow Up Work

Populate root/commitment fields

[codecov]

Codecov Report

Merging #5659 (8ef27df) into main (ea21e64) will increase coverage by 0.02%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5659      +/-   ##
==========================================
+ Coverage   78.69%   78.71%   +0.02%     
==========================================
  Files         306      306              
  Lines       38552    38552              
==========================================
+ Hits        30340    30348       +8     
+ Misses       8212     8204       -8     

[oxarbitrage]

zebra_rpc_conflict failing https://github.com/ZcashFoundation/zebra/actions/runs/3493963607/jobs/5849620107#step:3:9825

Retrying.

[arya2]

This looks great! 🎉

[teor2345]

I think we're almost there, just need to deal with some tricky edge-cases with time and difficulty on testnet.

[oxarbitrage]

Some comments that i am not totally sure for the testnet consensus rule.

[oxarbitrage]

By now, we don't have any tests for testnet specific code under minimum difficulty block mining.

[teor2345]

Thanks for the changes, I think we're almost there.

I'm happy to help write these changes, but I need to finish the ZIP-317 work first. So feel free to keep making changes here, or ask me to submit a PR on top of this one.

[teor2345]

By now, we don't have any tests for testnet specific code under minimum difficulty block mining.

This existing test might help?

zebra/zebra-chain/src/work/difficulty/tests/vectors.rs

Lines 393 to 396 in c812f88

	/// Test that testnet minimum-difficulty blocks are valid
	#[test]
	#[spandoc::spandoc]
	fn testnet_minimum_difficulty() -> Result<(), Report> {

[teor2345]

Thanks, I think we're almost there, let me know if you want me to do the minimum difficulty "time" change.

[oxarbitrage]

Please apply any of the missing stuff and merge if you can. Thank you.

[teor2345]

Let's see if that works