allow passing callbacks to registerAdmin/registerPersonal

[icewind1991]

This can be used by an app to prevent having to determine what settings pages needs to be added if the result is never used

Adjusted the files_external settings page to use the new callback option, this saved 4 db queries for each request made

[mention-bot]

@icewind1991, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Xenopathic, @MorrisJobke and @georgehrke to be potential reviewers.

[icewind1991]

https://github.com/nextcloud/server/pull/5173/files?w=1 for reviewing the changes without phpstorm deciding that it really wants to reformat the code

[blizzz]

Can we defer it until #4890 is done? Probably it won't be necessary then at all.

[codecov]

Codecov Report

Merging #5173 into master will decrease coverage by <.01%.
The diff coverage is 35.13%.

@@ Coverage Diff @@## master #5173 +/- ##
============================================
- Coverage 54.14% 54.14% -0.01% - Complexity 22286 22295 +9 
============================================
Files 1379 1379 Lines 85335 85336 +1 Branches 1321 1321 ============================================
- Hits 46208 46207 -1 - Misses 39127 39129 +2

Impacted Files	Coverage Δ	Complexity Δ
lib/public/App.php	23.52% <ø> (ø)	8 <0> (ø)	⬇️
apps/files_external/lib/AppInfo/Application.php	0% <0%> (ø)	7 <4> (-1)	⬇️
lib/private/legacy/app.php	52.49% <37.14%> (-0.93%)	229 <2> (+10)
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)

[icewind1991]

superseded by #4890