interfaces/system-observe: allow clients to enumerate DBus connection names

[jdstrand]

This came up within the context of https://forum.snapcraft.io/t/ubuntu-core-dbus-interface-question/2138/3. While one could argue that this access should be in a different interface, I feel it makes sense to add to the existing system-observe, since this is similar to ps aux output (which system-observe allows), but for DBus.

[jdstrand]

Note the CI test failures were unrelated to this PR, so I restarted them hoping they were transient.

[zyga]

LGTM

[codecov-io]

Codecov Report

Merging #3928 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3928     +/-   ##
=========================================
+ Coverage   75.92%   76.02%   +0.1%     
=========================================
  Files         418      420      +2     
  Lines       36295    36436    +141     
=========================================
+ Hits        27556    27701    +145     
+ Misses       6806     6802      -4     
  Partials     1933     1933

Impacted Files	Coverage Δ
interfaces/builtin/system_observe.go	100% <ø> (ø)	⬆️
interfaces/core.go	98.97% <0%> (-1.03%)	⬇️
httputil/transport17.go	100% <0%> (ø)
httputil/redirect18.go	100% <0%> (ø)
dirs/dirs.go	98.29% <0%> (+0.21%)	⬆️
interfaces/repo.go	98.08% <0%> (+0.27%)	⬆️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️
interfaces/apparmor/backend.go	77.04% <0%> (+4.99%)	⬆️
httputil/useragent.go	85% <0%> (+7.5%)	⬆️

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[jdstrand]

Thanks!