confdb,asserts,daemon: add confdb-control api

[st3v3nmw]

This PR introduces the confdb-control API to snapd.

Please do not review just yet. Awaiting:

• o/devicestate,asserts: sign confdb-control assertions with the device key #14723
• Final approval on the spec (I don't expect much to change given how the last design session went)

[codecov]

Codecov Report

Attention: Patch coverage is 98.99666% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.29%. Comparing base (24a0034) to head (8ccd304).
Report is 112 commits behind head on master.

Files with missing lines	Patch %	Lines
daemon/api_confdb.go	95.58%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14866      +/-   ##
==========================================
+ Coverage   78.20%   78.29%   +0.09%     
==========================================
  Files        1151     1157       +6     
  Lines      151396   153696    +2300     
==========================================
+ Hits       118402   120343    +1941     
- Misses      25662    25947     +285     
- Partials     7332     7406      +74     

Flag	Coverage Δ
unittests	78.29% <98.99%> (+0.09%)	⬆️

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[github-actions]

Thu Jan 16 15:42:52 UTC 2025
The following results are from: https://github.com/canonical/snapd/actions/runs/12809839545

Failures:

Preparing:

• openstack:debian-12-64:tests/regression/lp-2084730
• openstack:opensuse-15.6-64
• openstack:opensuse-tumbleweed-64:tests/main/snapshot-users

Executing:

• openstack:debian-12-64:tests/main/appstream-id
• openstack:debian-sid-64:tests/main/refresh-amend:try_mode
• google-distro-1:debian-11-64:tests/main/confdb
• google:ubuntu-25.04-64:tests/main/lxd-try

Restoring:

• openstack:debian-12-64:tests/regression/lp-2084730
• openstack:opensuse-15.6-64