Improve Apprise notification machinery. Add apprise_multi plugin.

[amotl]

Hi there,

based on #557, it brings notifications using Apprise closer to what @psyciknz was requesting at #555 (comment).

The idea behind this variant is to publish messages to different Apprise plugins within a single configuration snippet, containing multiple recipients in form of regular mqttwarn targets.

A corresponding configuration snippet would look like:

[defaults]
launch    = apprise-multi

[config:apprise-multi]
; Dispatch message to multiple Apprise plugins.
module   = 'apprise_multi'
targets = {
   'demo-http'        : [ { 'baseuri':  'json://localhost:1234/mqtthook' }, { 'baseuri':  'json://daq.example.org:5555/foobar' } ],
   'demo-discord'     : [ { 'baseuri':  'discord://4174216298/JHMHI8qBe7bk2ZwO5U711o3dV_js' } ],
   'demo-mailto'      : [ {
          'baseuri':  'mailtos://smtp_username:[email protected]',
          'recipients': ['[email protected]', '[email protected]'],
          'sender': '[email protected]',
          'sender_name': 'Example Monitoring',
          } ],
   }

[apprise-multi-test]
topic    = apprise/multi/#
targets  = apprise-multi:demo-http, apprise-multi:demo-discord, apprise-multi:demo-mailto

With kind regards,
Andreas.

[codecov-commenter]

Codecov Report

Merging #558 (1c7c43d) into main (b927a61) will increase coverage by 0.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
+ Coverage   28.12%   28.89%   +0.77%     
==========================================
  Files          81       83       +2     
  Lines        3677     3717      +40     
==========================================
+ Hits         1034     1074      +40     
  Misses       2643     2643              

Flag	Coverage Δ
unittests	28.89% <100.00%> (+0.77%)	⬆️

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Impacted Files	Coverage Δ
mqttwarn/model.py	100.00% <100.00%> (ø)
mqttwarn/services/apprise.py	100.00% <100.00%> (ø)
mqttwarn/services/apprise_multi.py	100.00% <100.00%> (ø)
mqttwarn/services/apprise_single.py	100.00% <100.00%> (ø)

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