feat: add admin routes for contract tasks

[JoE11-y]

Closes #267

• Create a contract folder in src\endpoints\admin containing a create_contract and an update_contract route (such as in src\endpoints\admin\balance), which are dedicated to creating & updating contract tasks.
• It should take a calls (Array of Call) field as an argument in the body (among the other default fields such as the task name), and store it in the task document in the db (as we are doing for other task types, such as in src\endpoints\admin\balance.).

[Marchand-Nicolas]

Working perfectly! Lgtrm!