Custom callout boxes

Firstly, many thanks for the fantastic package you are developing!

I am following from @athulsudheesh's comment in #556, to request a new feature for custom callout boxes.

One use case I have in mind is for exercises in books or course materials, for example:

:::{.callout-exercise}

Exercise description.

:::{.callout-answer collapse="true"}
The answer to the exercise
:::

:::

I guess these would have to be defined somewhere following a template (on the front matter? or on a separate YAML, that is then referenced on the front-matter?).

callout: "exercise"
    title: "Exercise"
    color: "purple"
    icon: "fa-question"
    collapse: "false"
callout: "answer"
    title: "Answer"
    color: "green"
    icon: "fa-pen"
    collapse: "true"    

Alternatively maybe this could be done through a generic callout with a couple of extra options, such as:

:::{.callout-custom color="purple" icon="fa-question"}
## Exercise

A purple callout block with a question mark icon.
:::

The disadvantage here is that there is more typing involved.

Review the existing callout behavior and the discussion in #556, including the linked comment. Compare the proposed custom syntax and YAML configuration examples, then determine the supported design and verify that custom exercise and answer callouts render with their requested titles, colors, icons, and collapse behavior.