feat: add mutex for locking subscriber map

[t-bre]

Description

When users call rtcan_subscribe, the subscriber hashmap is modified. If this happens while the RTCAN receive thread is running, it may not be thread safe.

Requirements

• Add a new mutex to RTCAN instances.
• Lock the mutex when rtcan_subscribe is modifying the subscriber hashmap.
• Lock the mutex when the RTCAN receive thread is reading the subscriber hashmap.