[NU-1977] Key variable change in window components

[ForrestFairy]

Describe your changes

key variable was created in window components by taking values from groupBy and parsing them to a list in string. Now it will be passed as a value which was provided, eg.

for id = 1
Before -> groupBy: id -> key: "[1]"
After -> groupBy: id -> key: 1

For old scenarios users would need to go into aggregation components and edit groupBy field for it to take effect. Otherwise it will still create key variable as string

Checklist before merge

• Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• Code is cleaned from temporary changes and commented out lines
• Parts of the code that are not easy to understand are documented in the code
• Changes are covered by automated tests
• Showcase in dev-application.conf added to demonstrate the feature
• Documentation added or updated
• Added entry in Changelog.md describing the change from the perspective of a public distribution user
• Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• Verify that PR will be squashed during merge

[raphaelsolarski]

Let's write what it is now, and add a little note to the migration guide.

[raphaelsolarski]

IMO we should wrap keys in list if there is more than one key (now the widget works odd if we put multiple key)

[raphaelsolarski]

[raphaelsolarski]

Shouldn't we type the key output variable according to type put in groupBy parameter? e.g. String or List[String].

[raphaelsolarski]