Test storing custom types

[julienbrs]

Issue: related to #70

Description

This PR introduces unit tests for storing_custom_type. The main objective of these tests is to demonstrate the storage of custom types within the contract's storage system. Currently, the custom type used does not implement the Serde trait. As a result, we cannot retrieve a return value from the get_person function, which limits its utility. I am considering removing this function due to its limited functionality.

[julio4]

I've made the following changes:

• Modified the get_person method to return Person.
• Altered tests to use ContractState instead of deployment.

The use of contract state is up for discussion. My rationale is that our goal is to demonstrate the syntax, making it unnecessary to directly test deployment.

[julienbrs]

I've made the following changes:

* Modified the `get_person` method to return `Person`.

* Altered tests to use `ContractState` instead of deployment.

The use of contract state is up for discussion. My rationale is that our goal is to demonstrate the syntax, making it unnecessary to directly test deployment.

Thanks for the review, it indeed makes sense. Following your changes, do we still need the get_person function? It is not used in the test and its usage is already shown in the custom_type_serde example.
I've pushed a version without it, feel free to choose which version to merge.