Improved async/threading attachment support

[caronc]

Description:

Related issue (if applicable): BirdNET-Pi/1171

Issue was detected in another system. Situation is a racing condition where multiple services want to share the same retrieved attachment. The problem is the asynchronous process starts all fetching at the same time. The ideal approach is to fetch the image once, and the remaining services share from the single fetch.

This PR Resolves this issue.

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

Anyone can help test this source code as follows:

# 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@/improved-attachment-support

# Test out the changes by just sending a web based attachment to 2 or more endpoints:
apprise -t "Test Title" -b "Test Message" \
  discord://credentials \
  pover://credentials \
 --attach=http://website/remote/image.jpg
  

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.28%. Comparing base (195d0ef) to head (e9f6ad3).

❗ Current head e9f6ad3 differs from pull request most recent head f14e764. Consider uploading reports for the commit f14e764 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1086   +/-   ##
=======================================
  Coverage   99.28%   99.28%           
=======================================
  Files         137      137           
  Lines       17874    17885   +11     
  Branches     3649     3652    +3     
=======================================
+ Hits        17746    17757   +11     
  Misses        119      119           
  Partials        9        9           

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