Add 'gdal raster nodata-to-alpha'

[rouault]

Fixes #12524

[jratike80]

Could it have an option to write an 1-bit mask band instead of alpha?

[rouault]

Could it have an option to write an 1-bit mask band instead of alpha?

I indeed considered there are other similar cases like nodata to mask band, mask band to alpha, alpha to mask band. All those can be accomplished with "gdal raster select" by using "-b {number|mask}" and "-mask {number|mask}".

[coveralls]

coverage: 71.065%. remained the same when pulling af7f56a02223b5356ad2c9fc7cd1434605eaa8ba on rouault:gdal_raster_nodata_to_alpha into d11b3d50ddcc9710522ff5e1d0c32119a48e3be9 on OSGeo:master.