Python: Add extractor flag telemetry

Records any non-default extractor flags (without their arguments) as a
normalised string. This will enable us to determine which flags are
actually used (and which ones we might therefore get rid of).

When there are no flags other than the ones the autobuilder injects, we
simply report the string `"default"`. That way, there's no need to
remember exactly which flags are enabled by default during extraction.