feat: add a 1band + alpha dataset

[vincentsarago]

This PR adds a Red+Alpha dataset in order to tests it in async-geotiff 👇

import rasterio
with rasterio.open("uint8_1band_and_alpha_deflate_block64_cog.tif") as src:
    print(src.colorinterp)
(<ColorInterp.red: 3>, <ColorInterp.alpha: 6>)

from async_geotiff import GeoTIFF
from obstore.store import LocalStore
store = LocalStore(".")
geotiff = await GeoTIFF.open("uint8_1band_and_alpha_deflate_block64_cog.tif", store=store)
geotiff.colorinterp
(<ColorInterp.GRAY: 1>, <ColorInterp.GRAY: 1>)

[kylebarron]

lgtm!