outposts: allow externally managed SSH Config for outposts

[BeryJu]

closes #2916

[codecov]

Codecov Report

Merging #2917 (1dd032a) into master (d9d4202) will decrease coverage by 0.02%.
The diff coverage is 27.28%.

@@            Coverage Diff             @@
##           master    #2917      +/-   ##
==========================================
- Coverage   91.81%   91.80%   -0.01%     
==========================================
  Files         460      460              
  Lines       20301    20307       +6     
==========================================
+ Hits        18637    18640       +3     
- Misses       1664     1667       +3     

Impacted Files	Coverage Δ
authentik/outposts/docker_ssh.py	34.49% <25.00%> (-0.06%)	⬇️
authentik/outposts/controllers/docker.py	81.58% <28.58%> (-1.30%)	⬇️
authentik/core/api/sources.py	82.90% <0.00%> (+2.64%)	⬆️

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