Add relposheading in process fix uavcan driver (AP Periph Dronecan Dual RTK support)

[dirksavage88]

Solved Problem

Adds support for dual RTK heading via AP Periph, which publishes heading as a distinct message (RelPosHeading) instead of via the ecef position fields.

Fixes #22520

Solution

• Add sub callback for relposheading within GNSS sensor uavcan driver
• Process heading information and validity in process fixx method

Changelog Entry

For release notes:

Feature: Add support for AP Periph (dronecan) dual GPS heading setups
New parameter: None
Documentation: Pending doc update PR in PX4 user guide

Alternatives

Create a sub to the sensor gnss relative uORB topic directly in EKF2

Test coverage

• Tested using dual Holybro ZED-F9P dronecan nodes and pixhawk 6xrt

Context

TODO: post log Pending doc update PR: https://github.com/PX4/PX4-user_guide/pull/3406