Allow planetary_computer.sign_url to refresh token of signed URLs

[bmcandr]

The sign_url function checks whether SAS token query parameters are present and, if present, returns the URL unaltered:

planetary-computer-sdk-for-python/planetary_computer/sas.py

Lines 161 to 164 in 0c1d784

	parsed_qs = parse_qs(parsed_url.query)
	if set(parsed_qs) & {"st", "se", "sp"}:
	# looks like we've already signed it
	return url

Is it possible to allow this function to refresh the token of signed URLs by checking the expiration datetime of the se query parameter?