Exercise on the output builtin

[tekkac]

Description

Add an exercise in the builtin tracks about output processing using the output_ptr variable and serializing functions.
Find a way to design a Cairo test to verify the output is effectively assigned.

Resource:

• builtins doc
• output doc
• serialize.cairo

Acceptance criteria

• Add an exercise in the builtins track folder on the output builtin
• The exercise compiles and tests pass
• An exercise patch is generated in .patch/
• The exercise list in src/exercises/__init__.py is updated