dulwich: use SubprocessSSHVendor when necessary

[pmrowla]

Falls back to using SubprocessSSHVendor instead of AsyncSSHVendor when:

• we are inside git-bash/msys environment on windows
• OpenSSH ~/.ssh/config exists and contains options which are unsupported by asyncssh

Fixes: treeverse/dvc#7702
Related: treeverse/dvc-ssh#20

Closes #208
Closes #210

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.93750% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.35%. Comparing base (3a1df25) to head (a54dcf4).
⚠️ Report is 212 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scmrepo/git/backend/dulwich/asyncssh_vendor.py	73.52%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
+ Coverage   82.29%   82.35%   +0.05%     
==========================================
  Files          25       25              
  Lines        3497     3559      +62     
  Branches      608      622      +14     
==========================================
+ Hits         2878     2931      +53     
- Misses        527      532       +5     
- Partials       92       96       +4     

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