feat(rpc): introduce getblocktemplate rpc call with stub fields

[oxarbitrage]

Motivation

This is pretty much what i have in mind for #5451

It introduces the rpc call and all the fields using mostly strings as placeholders. All data will be filled in the other tickets, this is kind of the skeleton.

This is a draft, feel free to suggest changes.

Solution

Introduce getblocktemplate rpc methjod, return empty GetblockTemplate struct on call.

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 Blocktemplate responses with fixed values data #5452
• Populate BlockTemplate fields using a new state request #5455
• Populate transparent coinbase transaction data for BlockTemplate #5453
• Populate transactions of BlockTemplate #5454

[codecov]

Codecov Report

Merging #5462 (575e89d) into main (95517bd) will increase coverage by 0.11%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5462      +/-   ##
==========================================
+ Coverage   79.10%   79.21%   +0.11%     
==========================================
  Files         308      309       +1     
  Lines       39324    39351      +27     
==========================================
+ Hits        31107    31172      +65     
+ Misses       8217     8179      -38     

[teor2345]

Looks good, but I think longpollid is out of scope for now.

[teor2345]

I wonder if we want to use underscores to match the rest of the RPC implementations.

[teor2345]

Looks good!

I think this is a high priority, because it renames the module that all the other RPCs are in. If we don't get it merged soon, it will cause code conflicts.