Raster symbology: Support flag values (e.g. missing data) with color ramps

[mfisher87]

Users should be able to apply a color ramp to their data and also specify values that should be colored uniquely, e.g. value -9999 for data that is missing might be rendered as bright red while the rest of the data is colored in viridis. "Missing" can be different than "nodata" (e.g. nodata is pixels where the satellite didn't pass over, and missing data could be missing for any reason) so we would need to enable the user to define multiple custom flag values.