Add `payload_init=` argument to `lib.stream.Signature()`

[whitequark]

Fixes #1404.

[whitequark]

Drafted pending discussion in #1404.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.06%. Comparing base (42d90a3) to head (47df939). Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1434      +/-   ##
==========================================
+ Coverage   90.83%   91.06%   +0.22%     
==========================================
  Files          43       43              
  Lines       10946    10967      +21     
  Branches     2305     2375      +70     
==========================================
+ Hits         9943     9987      +44     
+ Misses        830      805      -25     
- Partials      173      175       +2     

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[whitequark]

The PR still needs a documentation update.