[add] ntfy.sh notification support

[aidan-]

Motivation for changes:

User in #3445 requested support for ntfy.sh - it seems like a decent zero-config notification option for people wanting a simple solution.

Detailed changes:

• Add support for ntfy.sh notifications.

Implemented feature requests:

• Feature request: support for ntfy.sh #3445

Config usage if relevant (new plugin or updated schema):

tasks:
  seen1:
    mock:
      - { "title": "Big.Buck.Bunny.1080p.BluRay.x264-GRP", "url": "mock://local1" }
    accept_all: yes
    notify:
      entries:
        title: Download from task {{task}}
        template: message.template
        via:
          - ntfysh:
              url: "https://ntfy.sh/"
              topic: "flexget-test-topic"
              priority: 5
              delay: 30s
              tags: poop,train

Questions

Some of the notification implementations have this line in the example:

Configuration parameters are also supported from entries (eg. through set).

I couldn't find an example of how this is done. Can provider specific configuration options be set via the entry somehow?

[gazpachoking]

Thanks!