daemon: fetch validation sets before checking refresh candidates

[miguelpires]

snap refresh --list didn't report snaps that would be refreshed with a snap refresh because it, unlike the actual refresh, didn't refresh the validation sets which could make new snap revisions available. This change fetches new validation sets (without tracking them) before checking for refreshable snaps

[miguelpires]

@pedronis this PR adds a side-effect to snap refresh --list so that we don't have to add code to fetch assertions w/o committing them and make the refresh candidate checking code aware of arbitrary validation sets that aren't in the DB. I think it's a fair trade-off for a much less complex implementation but since that was a concern you had, I'm just pinging you here so you can veto it if you disagree

[andrewphelpsj]

Thank you!

[codecov]

Codecov Report

Attention: Patch coverage is 75.47170% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.22%. Comparing base (24a0034) to head (2c18d84).
Report is 92 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/assertstate/assertstate.go	83.33%	4 Missing and 3 partials ⚠️
daemon/api_find.go	45.45%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14907      +/-   ##
==========================================
+ Coverage   78.20%   78.22%   +0.02%     
==========================================
  Files        1151     1158       +7     
  Lines      151396   152944    +1548     
==========================================
+ Hits       118402   119645    +1243     
- Misses      25662    25912     +250     
- Partials     7332     7387      +55     

Flag	Coverage Δ
unittests	78.22% <75.47%> (+0.02%)	⬆️

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[bboozzoo]

LGTM

[miguelpires]

Failures:

openstack:fedora-41-64: cannot allocate new Openstack server jan081602-652568: server status is ERROR
openstack:centos-9-64: ... same
ubuntu-20.04-64:tests/unit/go:clang known issue