many: support creating recovery systems with components from the store

[andrewphelpsj]

This PR allows us to create recovery systems that contain components using the /v2/systems API.

[codecov]

Codecov Report

Attention: Patch coverage is 54.77273% with 199 lines in your changes missing coverage. Please review.

Project coverage is 78.27%. Comparing base (24a0034) to head (f5b4456).
Report is 73 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/lib/fakestore/refresh/snap_asserts.go	0.00%	79 Missing ⚠️
tests/lib/fakestore/store/store.go	67.94%	35 Missing and 15 partials ⚠️
overlord/devicestate/devicestate.go	81.66%	15 Missing and 7 partials ⚠️
...estore/cmd/fakestore/cmd_new_snap_resource_pair.go	0.00%	18 Missing ⚠️
...re/cmd/fakestore/cmd_new_snap_resource_revision.go	0.00%	18 Missing ⚠️
overlord/devicestate/systems.go	77.08%	8 Missing and 3 partials ⚠️
snap/snaptest/snaptest.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14883      +/-   ##
==========================================
+ Coverage   78.20%   78.27%   +0.07%     
==========================================
  Files        1151     1148       -3     
  Lines      151396   152936    +1540     
==========================================
+ Hits       118402   119714    +1312     
- Misses      25662    25842     +180     
- Partials     7332     7380      +48     

Flag	Coverage Δ
unittests	78.27% <54.77%> (+0.07%)	⬆️

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