feat: add ability to unsubscribe

[t-bre]

Description

Add the ability to unsubscribe from a CAN message.

Requirements

• Create rtcan_unsubscribe() function in public API.
• Remove subscriber entry from subscriber hashmap.
• Consume any messages in the subscription queue which match the given CAN ID, or delegate this responsibility to the user in some way.

[t-bre]

This will probably be needed by sufst/vcu when implementing its configuration protocol.