[NU-1701] Introduce extension methods to SpeL. Add canCastTo and cast…

[lukasz-bigorajski]

…To to SpeL.

Describe your changes

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