[NU-1836] Add from string conversions as SpeL extension methods

Describe your changes

[ ] 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