Shortcodes: Add Milestone Shortcode (or Gutenberg Block)

#### Is your feature request related to a problem? Please describe.
Currently, the milestone widget can only be used in the sidebar, as it's a widget: https://jetpack.com/support/extra-sidebar-widgets/milestone-widget/

#### Describe the solution you'd like
Like many widgets by Jetpack, it'd probably be suited as a shortcode too. That would allow milestones to be added to posts and pages, which would be a really handy feature.

I imagine that it'd just be `[milestone]` with an option to customise the date, title, message and event.

#### Describe alternatives you've considered
WPcom site owner here so options are limited :frowning_face: but it does seem odd to offer other widgets as shortcodes yet not this one. Bonus if it's a Gutenberg block, but I imagine that won't happen anytime soon.

```[tasklist]
### Tasks
```

Start by locating the existing Jetpack milestone widget and the other widgets exposed as shortcodes. Determine whether the project should add a [milestone] shortcode or a Gutenberg block, including configurable date, title, message, and event fields. Done means milestones can be inserted into posts and pages with those options.

Written by the indexing model from the issue text.