Add custom sound support to pushover

[cdkl]

Description:

Pushover integration constrains notification sounds to a predefined list. This change adds support for custom sound in notifications, which must be uploaded and given a name. This change updates the pushover integration to allow for that name to be specified instead of throwing an error.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

(I performed these steps)

1. Go to Settings -> Alert Settings -> Manage custom sounds -> Upload a sound
2. Upload a sound and specify a name (e.g. "mysound").
3. Validate the sound is accessible and present in the sounds list for your app via https://api.pushover.net/1/sounds.json?token={app-token}
4. Specify a sound in your pover call, i.e. apprise -vv -t "title" -b "test message" pover://user@app?sound=mysound

You should hear your custom sound on the notification. In cases where the custom sound name is not found, the default pushover notification sound will play.

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@<this.branch-name>

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (6458ab0) 100.00% compared to head (ea5e095) 99.98%.

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Additional details and impacted files

@@             Coverage Diff             @@
##            master     #843      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files          118      118              
  Lines        15356    15692     +336     
  Branches      3146     3207      +61     
===========================================
+ Hits         15356    15690     +334     
- Misses           0        1       +1     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
apprise/plugins/NotifyPushover.py	100.00% <100.00%> (ø)

... and 65 files with indirect coverage changes

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