In the marker schema, add the ability to mark fields as privacy-sensitive

We currently have two marker field formats called `string` and `sanitized-string`.

I think we should remove `sanitized-string` and allow marking any field, regardless of field format, as privacy-sensitive.

This would avoid the need to add a `sanitized-unique-string` format; instead you'd be able to just have a `unique-string` marked with the new field.

Another example of a privacy-sensitive field is the field that stores the screenshot image on CompositorScreenshot markers. CompositorScreenshot markers are currently handled manually during sanitization.

Yet another example are native key events: You might have a marker which carries an integer value with the native key code, which can be used to compute the pressed key. This integer would then be privacy-sensitive, because it would allow you to compute the text that the user typed into a textbox.

---

Proposal:

```js
type PrivacyCategory = "screenshot" | "url" | "local-path" | "keylogger-ingredient";

type MarkerFieldBase = {
...,
isPrivacySensitiveWithCategory?: PrivacyCategory,
}
```

The privacy category would determine which checkbox in the upload panel should cause the field to be sanitized away.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-851)