Add --export-autocrypt-key command to rnpkeys CLI and a test.

[kaie]

• Adds --export-autocrypt-key command to rnpkeys, producing Autocrypt-format output via rnp_key_export_autocrypt()
• Fixes rnp_key_export_autocrypt not setting output->keep = true on success, which caused rnp_output_destroy to delete the output file
• Adds rnpkeys_exportkey_autocrypt test covering round-trip export/import

[codecov]

Codecov Report

❌ Patch coverage is 54.16667% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.39%. Comparing base (83a987a) to head (66f6786).

Files with missing lines	Patch %	Lines
src/rnp/fficli.cpp	58.53%	17 Missing ⚠️
src/rnpkeys/rnpkeys.cpp	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2386      +/-   ##
==========================================
- Coverage   85.46%   85.39%   -0.07%     
==========================================
  Files         126      126              
  Lines       22711    22759      +48     
==========================================
+ Hits        19409    19435      +26     
- Misses       3302     3324      +22     

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