Add backend expressions to expression.schema.v1.json

### Description

**Background:**
At Team Studio, we utilize expression.schema.v1.json to validate the construction of expressions and ensure their validity. This works excellently for all front-end expressions. However, as we make the configure processes and expressions that run on the backend, it becomes necessary to incorporate backend expressions into this schema. Without this integration, the StudioExpression component will fail to validate the expression accurately.

**Solution Proposal:**
We propose expanding expression.schema.v1.json to include backend expressions and adding an error message to the app-frontend if anyone attempts to configure a backend expression in the frontend.

### Additional Information
https://docs.altinn.studio/altinn-studio/reference/process/flowcontrol/

Start by locating expression.schema.v1.json and the StudioExpression component in app-frontend-react, then compare the schema with the backend expressions described in the linked flow-control documentation. Done means the schema validates backend expressions and the frontend displays an error when someone tries to configure one there.

Written by the indexing model from the issue text.