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 67.56757% with 96 lines in your changes missing coverage. Please review.

Project coverage is 78.29%. Comparing base (24a0034) to head (1ed5ca7).
Report is 74 commits behind head on master.

Files with missing lines	Patch %	Lines
daemon/api_confdb.go	12.90%	54 Missing ⚠️
confdb/confdb_control.go	83.04%	24 Missing and 5 partials ⚠️
asserts/confdb.go	79.36%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14866      +/-   ##
==========================================
+ Coverage   78.20%   78.29%   +0.08%     
==========================================
  Files        1151     1150       -1     
  Lines      151396   152819    +1423     
==========================================
+ Hits       118402   119647    +1245     
- Misses      25662    25800     +138     
- Partials     7332     7372      +40     

Flag	Coverage Δ
unittests	78.29% <67.56%> (+0.08%)	⬆️

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