decodeBase64Url
Decodes this string using the url-safe base64 alphabet of RFC 4648 section 5.
Both padded and unpadded input is accepted. When padding is present, the total length must be a multiple of four and at most two = characters may appear, only at the end. The standard characters + and / are rejected. An empty string produces an empty array.
Return
the decoded bytes.
Throws
if the input contains a character outside the url-safe alphabet, if padding is malformed, or if the number of data characters leaves a remainder of one when divided by four.