Enable header parsing to use R: futures::io::AsyncBufRead

[str4d]

Part of #368.

[codecov]

Codecov Report

Merging #393 (37012ba) into main (60c3946) will increase coverage by 0.50%.
The diff coverage is 56.03%.

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   40.45%   40.96%   +0.50%     
==========================================
  Files          34       34              
  Lines        3181     3237      +56     
==========================================
+ Hits         1287     1326      +39     
- Misses       1894     1911      +17     

Impacted Files	Coverage Δ
age/src/cli_common.rs	13.97% <ø> (ø)
age/src/scrypt.rs	57.77% <0.00%> (-0.92%)	⬇️
age/src/primitives/armor.rs	42.11% <42.85%> (+0.72%)	⬆️
age/src/format.rs	51.26% <46.66%> (-0.67%)	⬇️
age/src/protocol.rs	68.02% <100.00%> (+1.35%)	⬆️
age/src/ssh/identity.rs	61.86% <100.00%> (-0.33%)	⬇️
age/src/ssh/recipient.rs	64.28% <100.00%> (-0.63%)	⬇️
age/src/x25519.rs	80.64% <100.00%> (-0.61%)	⬇️
age/tests/testkit.rs	89.07% <100.00%> (+1.82%)	⬆️

... and 3 files with indirect coverage changes