Add FormDataEncoder.encode(_:,boundary:,into:)

[ptoffy]

Add FormDataEncoder.encode method that takes an existing buffer to encode into

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.48%. Comparing base (6a93d5c) to head (5d992c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   84.31%   84.48%   +0.16%     
==========================================
  Files          27       27              
  Lines        1014     1025      +11     
==========================================
+ Hits          855      866      +11     
  Misses        159      159              

Files with missing lines	Coverage Δ
...MultipartKit/FormDataEncoder/FormDataEncoder.swift	100.00% <100.00%> (ø)
...es/MultipartKit/Writer/MemoryMultipartWriter.swift	100.00% <100.00%> (ø)

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