feat(msp_displayport): add MSP_DP_SYS support for VTX temperature

[theogayar]

This PR implements MSP DisplayPort Subcommand 6 (MSP_DP_SYS) to allow flight controllers (especially Betaflight) to request rendering of the VTX temperature.

Technical changes:

• Added SUBCMD_SYS (6) and DISPLAYPORT_SYS_VTX_TEMP (0) enums to msp_displayport.h.
• Added SUBCMD_SYS handler to parse_displayport() in msp_displayport.c.
• Implemented a lightweight conversion for raw temperature (temperature >> 2).
• Updated OSD character buffer (osd_buf) and location bitfield (loc_buf) at specified row and column coordinates.

Resolves #234

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Notes for Reviewers:

• Status: Marked as draft because it is untested on my hardware yet. Compiles without errors.
• Review needed: I'm new to this repository and low-level code in general, and after spending the time I'm beginning to question my choice of going down the rabbit hole. This is my take on a feature I wanted to try implementing, but it should absolutely be reviewed by someone with deeper knowledge. Feedback is highly appreciated!