feat: generate RFC json at publication time

[jennifer-richards]

Datatracker has taken over generating RFC JSON when RFC metadata are updated or when the RFC itself is obsoleted or updated by a new publication. This extends that to creating the initial JSON file.

After this change is deployed, Purple can stop sending its version of the JSON. To ease that transition, this continues to accept JSON but ignores the file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.59%. Comparing base (8dbc1bc) to head (4072d17).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11143      +/-   ##
==========================================
+ Coverage   88.52%   88.59%   +0.06%     
==========================================
  Files         334      333       -1     
  Lines       45102    44666     -436     
==========================================
- Hits        39927    39570     -357     
+ Misses       5175     5096      -79     

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