Federation fixes stable11

[schiessle]

fixes exchange of shared secret and user data among trusted servers

backport of #3825

[mention-bot]

@schiessle, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nickvergessen, @rullzer and @LukasReschke to be potential reviewers.

[codecov-io]

Codecov Report

Merging #4029 into stable11 will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@              Coverage Diff               @@
##             stable11    #4029      +/-   ##
==============================================
- Coverage        57.5%   57.49%   -0.01%     
- Complexity      19500    19510      +10     
==============================================
  Files            1211     1211              
  Lines           72678    72693      +15     
  Branches         1235     1235              
==============================================
+ Hits            41792    41798       +6     
- Misses          30886    30895       +9

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/CardDAV/SyncService.php	44.16% <5.26%> (-2.69%)	34 <7> (+6)
.../federation/lib/Middleware/AddServerMiddleware.php	86.66% <50%> (-5.65%)	4 <0> (+1)
apps/dav/lib/CardDAV/Converter.php	74.62% <88.88%> (+1.61%)	23 <0> (+3)	⬆️

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[rugk]

When will a release with this fix published?

[MariusBluem]

@rugk Next maintainance of Nextcloud 11 (11.0.3) was not announced yet, but you can expect it in the coming weeks.

[MorrisJobke]

@rugk Next maintainance of Nextcloud 11 (11.0.3) was not announced yet, but you can expect it in the coming weeks.

The plan is 10th April - https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule