fix: validate system accounts

[wei3erHase]

Fixes HAL-003

Description

In order to participate in a token locking process, a user needs to first invoke the process_init entry point.

This entry point will create an account with the necessary size to store the information provided in the next step of the process, process_create .

The current version of this program is not checking that the provided System Program and Rent Sysvar accounts corresponds to the correct accounts, provided by the Solana Rust SDK.

Although there was not risk identified due to this lack of restrictions, it is a good practice to implement this validations to prevent any unexpected attack vector that might appear later.