This adds the ability to store and load the Wi-Fi firmware for Pico 2 W in a partition. It can be enabled by adding pico_use_partition_firmware(<exe_name>) to your CMakeLists.txt, which will embed a compatible partition table in the binary, and output lots of firmware UF2s to use (all called <exe_name>_firmware_..._.uf2, eg ..._firmware_w for Wi-Fi only, ..._firmware_wb for Wi-Fi and Bluetooth, ..._firmware_w_tbyb for TBYB). You can also create your own partition table and use that.

A Wi-Fi firmware partition is detected as having the ID 0x123456789abcdef0, and the UF2 family_is for Wi-Fi firmware blobs is 0x12345678 - these should probably both be changed to something else before merging? The default firmware partition starts at 3500K into the flash - should this be changed to depend on PICO_FLASH_SIZE_BYTES? The default firmware partition is also duplicated with A/B partitions in the same location in flash - this is required to ensure a signature check is performed before loading the Wi-Fi firmware, as there's no way to call the bootrom to check the signature of a single partition (unless chaining into it), you can only call pick_ab_partition.

The Wi-Fi firmware blob is marked in it's image_def as an RP2350 Risc-V executable, and the partition is marked as ignored_during_riscv_boot - this ensures that it can work with TBYB (as TBYB only works for executable image_defs), and that signature checks are performed before loading the firmware when Secure Boot is enabled (because signature checks are performed for all executable image_defs in a partition that is not marked ignored_during_arm_boot). This is slightly clunky, but seems to work robustly.

Supercedes #1850, as it now includes that function in this PR, and shows a use for it.