feat(docs): move l1asset smart contract example from wiki to wasp

[Ginowine]

Description of change

This PR moves the L1Assets.sol smart contract from the wiki to the Wasp repo, alongside a solo test for it.

Links to any relevant issues

`fixes issue iotaledger/iota-wiki#1615

How the change has been tested

A solo test

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have selected the develop branch as the target branch
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[lucas-tortora]

could you rename the PR to follow the guidelines on confluence?