client: add support to send notices

[ZeyadYasser]

This is a backport from pebble: canonical/pebble#297 to allow sending notices from snap command.

This is a building block for #13770 to allow snap run to record snap-run-inhibit commands.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.72727% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 78.85%. Comparing base (ab996a0) to head (58c854d).

Files	Patch %	Lines
client/notices.go	72.72%	4 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13791      +/-   ##
==========================================
- Coverage   78.86%   78.85%   -0.01%     
==========================================
  Files        1043     1044       +1     
  Lines      134595   134617      +22     
==========================================
+ Hits       106144   106154      +10     
- Misses      21837    21845       +8     
- Partials     6614     6618       +4     

Flag	Coverage Δ
unittests	78.85% <72.72%> (-0.01%)	⬇️

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[zyga]

LGTM with one nitpick on documentation.

Do we need to document the public API endpoint on the forum?

[ZeyadYasser]

LGTM with one nitpick on documentation.

Do we need to document the public API endpoint on the forum?

This is intended to be only used through snap run, in fact we go through extra effort verify it is coming from snap run. I believe it is better to keep the endpoint undocumented until we add another generic notice type similar to custom notices in pebble.

[bboozzoo]

LGTM