Randomness on demand lesson as an update for Verifiable Randomness function lesson

[kmadorin]

Problem

The "Verifiable Randomness Functions" lesson currently uses the switchboard-v2 library, which is deprecated. Developers following this lesson may encounter outdated methods for generating randomness in Solana applications. The switchboard-on-demand library is now the recommended method for on-demand randomness, and there is no existing lesson covering this updated approach.

Summary of Changes

• Added a new lesson titled "Randomness On-Demand," which demonstrates the following:
  • How Randomness On-Demand works and how to set up and use the switchboard-on-demand library for requesting randomness in Solana smart contracts.
  • A practical example integrating on-demand randomness into Solana programs, reflecting the updates made to the burry-escrow program, which now uses the switchboard-on-demand library for randomness generation.

Fixes #

• randomness_on_demand.md file added to connecting-to-offchain-data courses