Allow variables plugin to replace arbitrary sections of config

[gazpachoking]

Motivation for changes:

Variables plugin could not be used for anything except for string fields in config (or parts of string fields.)

Detailed changes:

• Allows variables plugin to replace complex bits of config, like numbers, lists, ore whole dictionaries.
• Allows variables to be defined directly in the main config.

Addressed issues:

• Fixes Cannot use variables with integer-only values #1765

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

variables:
  mock_entry_list:
  - title: a
  - title: b
  integer: 2
tasks:
  test_int_var:
    limit_new: "{? integer ?}"
  test_var_mock:
    mock: "{? mock_entry_list ?}"

To Do:

• Update upgrade actions. Users that were relying on the string conversion will have to add some quotes to achieve old behavior.
• Make sure there aren't any other side effects. (adding extra quotes in replacement should guarantee old behavior, as noted in upgrade actions)

[liiight]

So you can basically use this now instead of yaml anchors ?
Also you had that native environment idea back in the day, would that fit this PR as well?

[gazpachoking]

So you can basically use this now instead of yaml anchors ?

Yeah, I think. Can't do any of the anchor dict merging stuff though.

Also you had that native environment idea back in the day, would that fit this PR as well?

This is that idea. 😉 It turns out it can do more than I thought.

[liiight]

This is that idea. 😉 It turns out it can do more than I thought.

Cool 😄