feat(upload): add generic UF2 uploader recipe

[cumin777]

Summary

This PR adds a generic Arduino upload recipe for boards that use a UF2 bootloader reached through a CDC 1200-bps touch and then exposed as a UF2 mass-storage volume.

This PR implements the following feature:

• Add the uf2upload tool definition and upload command in platform.txt.
• Breaking changes

The recipe keeps board-specific UF2 address, family ID, volume label, size limit, and timeout values in boards.txt, while sharing one uploader definition across boards.

Dependencies and split

• PR feat(xiao): add Seeed XIAO STM32C5 board support #3048 (XIAO STM32C5 board support) sets XIAO_STM32C5.upload.tool=uf2upload and supplies the board-specific UF2 parameters; this recipe is therefore required for that board's Arduino upload menu to work. The two PRs can be reviewed independently, but feat(xiao): add Seeed XIAO STM32C5 board support #3048's UF2 upload path is complete only after this recipe is merged.
• The recipe invokes the uf2upload executable supplied by Arduino_Tools PR Make Serialx instance generic #122. That tools PR must land first (or together), followed by a STM32Tools release/package-index update that makes the executable available to Arduino users.
• The recipe itself is independent of PR feat(usb): add STM32C5 HAL v2 USB device support #3059 (STM32C5 USB HAL v2 support), but end-to-end 1200-bps touch upload on the C5 requires the USB CDC support from feat(usb): add STM32C5 HAL v2 USB device support #3059.
• It is independent of the UART PR fix(uart): support STM32C5 HAL v2 reset and IRQ handling #3060.

Validation

• The change is limited to the generic tools.uf2upload.* definition in platform.txt.
• End-to-end upload validation depends on the Arduino_Tools Make Serialx instance generic #122 executable and its packaged STM32Tools release.

Code formatting

• AStyle formatting is not applicable to this platform configuration change; CI should verify the repository checks.

Closing issues

None.

[fpistm]

Thanks for the PR.
Where can we find the bootloader ?