feat: impersonator middleware feature/usability improvements

[leo9800]

this patch tackles 3 major issues of the current implementation of impersonator:

1. handling additional groups

unix users could retrieve groups they belongs with id command, it is common that a user belongs to multiple additional groups than the one which named identical to him/herself. here below is an example retrieving my groups on my linux workstation:

$ id
uid=1000(leo) gid=1000(leo) groups=1000(leo),150(wireshark),970(libvirt),971(docker),998(wheel)

the current implementation, wsgidav only call os.setegid(1000) when impersonating as leo on this machine, while other groups, incl. wireshark, docker, etc, should also be added.

this patch handles such cases with os.initgroups() to properly add additional groups accordingly (to /etc/group) when impersonating.

2. rejecting system users

uids <= 999 on unix systems are generally preserved for system daemon uses, a sysadmin may not want someone being capable to impersonate as such users. (probably due to misconfiguration of domain controller, etc)

this patch adds an option to reject impersonating-as-system-users attemps:

impersonator:
    reject_system_users: true

3. docker capability issues

docker's --cap-add (or cap_add: in docker-compose.yml) does not add the capabilities to the ambient set, therefore the capability is dropped upon fork()/execve(), (which is behind gunicorn's worker spawning) rendering the approach described in #343 (comment) not feasible.

this patch included a statically-linked helper program in C to achieve this in containers.

suid-wrapper nobody wsgidav -c /path/to/wsgidav.yaml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.12%. Comparing base (f12b2ec) to head (44a2fcd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   44.06%   44.12%   +0.06%     
==========================================
  Files          29       29              
  Lines        4893     4893              
==========================================
+ Hits         2156     2159       +3     
+ Misses       2737     2734       -3     

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