[image_picker] Endorse Windows implementation

[stuartmorgan-g]

flutter/plugins#4863 adds a partial, unendorsed Windows implementation. This tracks endorsing it once it's sufficiently complete:

• Support the camera source

Optional items that are currently missing, but I don't think should block endorsement:

• Support the image adjusting parameters (max size, quality)
  • Depending on whether there's an available system library that would let us do this without a lot of complexity (like having to bundle another library)
• Support max video duration

/cc @azchohfi