many: add a *SnapAppSet to ConnectedPlug/Slot types and use it to build label expressions in interfaces

[andrewphelpsj]

Now that we have app sets in the interfaces repo, keep a pointer to them in ConnectedPlug/Slot types. Use this to build label expressions in the interfaces. Based on #13772, first relevant commit is 4985f65.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.60735% with 214 lines in your changes are missing coverage. Please review.

Project coverage is 78.74%. Comparing base (1bad859) to head (db5f741).
Report is 15 commits behind head on master.

Files	Patch %	Lines
interfaces/ifacetest/backendtest.go	0.00%	64 Missing ⚠️
overlord/ifacestate/handlers.go	48.30%	42 Missing and 19 partials ⚠️
interfaces/ifacetest/app_set.go	38.46%	30 Missing and 2 partials ⚠️
overlord/ifacestate/helpers.go	45.71%	12 Missing and 7 partials ⚠️
interfaces/repo.go	61.70%	12 Missing and 6 partials ⚠️
interfaces/connection.go	33.33%	8 Missing ⚠️
interfaces/snap_app_set.go	93.24%	4 Missing and 1 partial ⚠️
interfaces/kmod/backend.go	71.42%	0 Missing and 2 partials ⚠️
daemon/api_sideload_n_try.go	0.00%	1 Missing ⚠️
interfaces/builtin/modem_manager.go	50.00%	1 Missing ⚠️
... and 3 more

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13773      +/-   ##
==========================================
- Coverage   78.83%   78.74%   -0.09%     
==========================================
  Files        1043     1044       +1     
  Lines      134470   135006     +536     
==========================================
+ Hits       106012   106314     +302     
- Misses      21847    22039     +192     
- Partials     6611     6653      +42     

Flag	Coverage Δ
unittests	78.74% <58.60%> (-0.09%)	⬇️

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