add pyfuse3 as an alternative lowlevel fuse implementation

[ThomasWaldmann]

No description provided.

[codecov-io]

Codecov Report

Merging #5430 into master will decrease coverage by 0.07%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##           master    #5430      +/-   ##
==========================================
- Coverage   83.69%   83.62%   -0.08%     
==========================================
  Files          37       38       +1     
  Lines        9957     9982      +25     
  Branches     1656     1661       +5     
==========================================
+ Hits         8334     8347      +13     
- Misses       1142     1151       +9     
- Partials      481      484       +3     

Impacted Files	Coverage Δ
src/borg/archiver.py	81.26% <33.33%> (-0.01%)	⬇️
src/borg/fuse_impl.py	84.61% <84.61%> (ø)
src/borg/archive.py	82.64% <0.00%> (-0.62%)	⬇️
src/borg/helpers/parseformat.py	89.16% <0.00%> (+0.16%)	⬆️

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[Gu1nness]

• I would add a test where BORG_FUSE_IMPL is set to an empty variable to ensure that it crashed the way we want to avoid regressions in the future.
• The documentation needs to be updated to be indented the way we should.
• I have run all the tests and it seems to be working (fuse2 and fuse3)

[ThomasWaldmann]

Squashed the fixup, reword another commit comment. Will merge after tests complete.