[feat] Add ntfy notification agent

[nwithan8]

Description

Added ntfy as configurable notification agent (virtually the same as gotify)

Example config:

ntfy:
  url: https://ntfy.sh  # Or your self-hosted instance
  topic: kometa
  token: tk_youruseraccesstoken
...
webhooks:
  # When PMM starts running
  run_start: ntfy
  # When PMM finishes running
  run_end: ntfy
  # When PMM encounters an error
  error: ntfy
  # When a change is made to a collection
  changes: ntfy
  # When a collection is deleted
  delete: ntfy
  # When there is a new version of PMM available
  version: ntfy

What users will see (desktop and mobile app):

ntfy has five different available priority levels:

• The testing message to verify access is sent at the lowest priority (1)
• New version available (2)
• Changes/deletions (3)
• Run start/end (4)
• Error sent at the highest priority (5)

Issues Fixed or Closed

• Fixes #(issue)

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation change (non-code changes affecting only the wiki)
• Infrastructure change (changes related to the github repo, build process, or the like)

Checklist

Please delete options that are not relevant.

• Updated Documentation to reflect changes
• Updated the CHANGELOG with the changes

[YozoraXCII]

I have tested this and can confirm that I receive notifications via ntfy:

I would propose adding some guidance to the docs that the user has to go to https://ntfy.sh/login and login/create an account, then go to the Account section (https://ntfy.sh/account) to get their token

[YozoraXCII]

Give me a shout here or on Discord once changes are made to reflect above comment and I can get this merged into nightly ahead of the upcoming release.