In response to https://github.com/rails/rails/issues/32917

In the current implementation, ActiveStorage passes all options to the underlying processor,
including when a key has a value of false.

For example, passing:

```
avatar.variant(resize: "100x100", monochrome: false, flip: "-90")
```

will return a monochrome image (or an error, pending on ImageMagick configuration) because
it passes `-monochrome false` to the command (but the command line does not allow disabling
flags this way, as usually a user would omit the flag entirely to disable that feature).

This fix only passes those keys forward to the underlying processor if the value responds to
`present?`. In practice, this means that `false` or `nil` will be filtered out before going
to the processor.

One possible use case would be for a user to be able to apply different filters to an avatar.
The code might look something like:

```
  variant_options = {
    monochrome: params[:monochrome],
    resize:     params[:resize]
  }

  avatar.variant(*variant_options)
```

Obviously some sanitization may be beneficial in a real-world scenario, but this type of
configuration object could be used in many other places as well.

- Add removing falsy values from varaints to changelog

- The entirety of #image_processing_transformation inject block was wrapped in `list.tap`
 to guard against the default `nil` being returned if no conditional was called.

- add test for explicitly true variant options

ImageProcessing gem is a wrapper around MiniMagick and ruby-vips, and
implements an interface for common image resizing and processing. This
is the canonical image processing gem recommended in [Shrine], and
that's where it developed from. The initial implementation was extracted
from Refile, which also implements on-the-fly transformations.

Some features that ImageProcessing gem adds on top of MiniMagick:

  * resizing macros
    - #resize_to_limit
    - #resize_to_fit
    - #resize_to_fill
    - #resize_and_pad
  * automatic orientation
  * automatic thumbnail sharpening
  * avoids the complex and inefficient MiniMagick::Image class
  * will use "magick" instead of "convert" on ImageMagick 7

However, the biggest feature of the ImageProcessing gem is that it has
an alternative implementation that uses libvips. Libvips is an
alternative to ImageMagick that can process images very rapidly (we've
seen up 10x faster than ImageMagick).

What's great is that the ImageProcessing gem provides the same interface
for both implementations. The macros are named the same, and the libvips
implementation does auto orientation and thumbnail sharpening as well;
only the operations/options specific to ImageMagick/libvips differ. The
integration provided by this PR should work for both implementations.

The plan is to introduce the ImageProcessing backend in Rails 6.0 as the
default backend and deprecate the MiniMagick backend, then in Rails 6.1
remove the MiniMagick backend.

Closes #32096.

If a variant has a large set of options associated with it, the generated
filename will be too long, causing Errno::ENAMETOOLONG to be raised. This
change replaces those potentially long filenames with a much more compact
SHA256 hash. Fixes #30662.

This fixes following warnings:

```
test/models/variant_test.rb:11: warning: ambiguous first argument; put parentheses or a space even after `/' operator
lib/active_storage/attached/macros.rb:63: warning: instance variable @active_storage_attached_highlights not initialized
lib/active_storage/attached/macros.rb:25: warning: instance variable @active_storage_attached_avatar not initialized
```