Add support for the u-Blox ZED-X20P GNSS module

## Feature request

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
Add support for the u-Blox ZED-X20P GNSS module in AP_GPS / UBLOX .

**Describe alternatives you've considered**
_A clear and concise description of any alternative solutions or features you've considered._

**Platform**
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Additional context**
This module is the latest version from u-Blox and it supports L1/L2/L5/L6 with RTK. It has current support for u-Blox Point Perfect corrections service (IP and L-band) and future support for in Q1 26 for centimeter positioning with Galileo High Accuracy services delivered via E6b.

Start by reviewing the AP_GPS / UBLOX implementation; the issue does not name a file, test, or entry point beyond those components. Determine the module-specific work needed for ZED-X20P support, and verify that the module works across the stated all-platform scope.

Written by the indexing model from the issue text.