repokey: use same format as with external keyfile

[ThomasWaldmann]

• always have a starting line with FILE_ID repoid
• store repkeys content-addressed, name is sha256(content)
• search by repo id on load
• add keyfile_format / keyfile_parse / is_keyfile helpers

[codecov]

Codecov Report

❌ Patch coverage is 76.66667% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.88%. Comparing base (a6124fc) to head (9d2dd1d).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/crypto/key.py	74.41%	7 Missing and 4 partials ⚠️
src/borg/repository.py	66.66%	7 Missing and 2 partials ⚠️
src/borg/crypto/keymanager.py	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9693      +/-   ##
==========================================
- Coverage   83.95%   83.88%   -0.08%     
==========================================
  Files          93       93              
  Lines       15586    15653      +67     
  Branches     2335     2351      +16     
==========================================
+ Hits        13086    13130      +44     
- Misses       1775     1789      +14     
- Partials      725      734       +9     

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