Fix spaces over federation

[S7evinK]

Fixes #2504

A few issues with the previous iteration:

• We never returned inaccessible_children, which (if I read the code correctly), made Synapse raise an error and thus not returning the requested rooms
• For restricted rooms, we didn't return the list of allowed rooms

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 49 lines in your changes are missing coverage. Please review.

Project coverage is 67.35%. Comparing base (ad0a7d0) to head (3d18878).

Files	Patch %	Lines
roomserver/internal/query/query_room_hierarchy.go	0.00%	41 Missing ⚠️
federationapi/routing/query.go	0.00%	4 Missing ⚠️
roomserver/api/wrapper.go	0.00%	3 Missing ⚠️
clientapi/routing/room_hierarchy.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3347      +/-   ##
==========================================
- Coverage   67.43%   67.35%   -0.08%     
==========================================
  Files         521      521              
  Lines       47348    47364      +16     
==========================================
- Hits        31928    31904      -24     
- Misses      11418    11460      +42     
+ Partials     4002     4000       -2     

Flag	Coverage Δ
unittests	52.75% <0.00%> (-0.02%)	⬇️

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[bones-was-here]

When running 3d18878 I can no longer reproduce issues with spaces over federation in Element Web. I tested:

• Synapse joining a space whose children contain no Synapses and whose m.space.child events list only Dendrite
• Dendrite joining a space whose children contain no Synapses and whose m.space.child events list only a different Dendrite
• Spaces containing a mix of public and invite-only rooms, using Dendrite and Synapse accounts that aren't members of the invite-only rooms (to make sure they didn't get listed)
• Spaces containing a mix of public and invite-only rooms, using Dendrite and Synapse accounts that are members of the invite-only rooms

[S7evinK]

Thanks a lot for testing confirming what I saw :)