Allow selection of tag in pico_extras_import.cmake

[apvanzanten]

The pico-sdk pico_sdk_import.cmake allows selection of a git tag to use when fetching the sdk code, via the PICO_SDK_FETCH_FROM_GIT_TAG variable (see https://github.com/raspberrypi/pico-sdk/blob/2.0.0/external/pico_sdk_import.cmake). This is quite useful if e.g. you want to keep using SDK 1.5.1 because your project is somehow not compatible with 2.0.0. Unfortunately, pico_extras_import.cmake does not provide this kind of customizability, it always just fetches from master. It would very nice if we could have this also in pico_extras_import.cmake.