interfaces: add Connection type

[stolowski]

Just a baby step to introduce new Connection struct, it's meant to be used universally across snapd.

[zyga]

Looks good

[codecov-io]

Codecov Report

Merging #3939 into master will increase coverage by 0.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #3939      +/-   ##
==========================================
+ Coverage   75.86%   75.88%   +0.01%     
==========================================
  Files         416      417       +1     
  Lines       36130    36197      +67     
==========================================
+ Hits        27410    27467      +57     
- Misses       6794     6804      +10     
  Partials     1926     1926

Impacted Files	Coverage Δ
interfaces/core.go	97.26% <0%> (-2.74%)	⬇️
interfaces/repo.go	97.8% <100%> (ø)	⬆️
interfaces/builtin/desktop.go	86.84% <0%> (-13.16%)	⬇️
overlord/snapstate/snapstate.go	79.85% <0%> (-0.15%)	⬇️
overlord/snapstate/handlers.go	65.53% <0%> (-0.07%)	⬇️
overlord/snapstate/snapmgr.go	82.29% <0%> (ø)	⬆️
release/release.go	100% <0%> (ø)	⬆️
apparmor/probe.go
release/apparmor.go	94.11% <0%> (ø)
interfaces/backends/backends.go	100% <0%> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c23401a...5b58270. Read the comment docs.