Allow for custom functions in layout expressions

### Background

More advanced logic for expressions (as detailed further in the [layout expressions project](https://github.com/orgs/Altinn/projects/22)) might require custom function app developers can implement themselves.

### Considerations
- Should we require implementations on both the backend and frontend? If the only implementation for a resource-heavy expression exists on the backend, we need to consider caching on the frontend. It could also be possible to implement custom function in the frontend only (as we already have in existing dynamic rendering)
- Strict typing, argument types, return types
- JSON schema support

No implementation files, tests, or entry points are named. Start with the linked layout expressions project, then resolve whether custom functions belong on the frontend, backend, or both, including caching, typing, and JSON schema support. Done should be an agreed design and implementation scope for custom functions.

Written by the indexing model from the issue text.