fix: try base64 legacy decoding for maximum compatibility

[amircybersec]

Currently Outline SDK config parser fails for strings encoded in legacy base64 mode such as the example below:

ss://YWVzLTEyOC1nY206c2hhZG93c29ja3M=@210.100.100.100:443

Current Outline client is however able to parse and decode the above URL. For consistent behaviour between Outline SDK and Outline client, I have added a secondary decoding step if current method fails.

Current aproach:

encoding := base64.URLEncoding.WithPadding(base64.NoPadding).DecodeString(userInfo)

Backup approach:

encoding := base64.StdEncoding.WithPadding(base64.NoPadding).DecodeString(userInfo)

[sbruens]

Do you mind adding a unit test for this case?

[amircybersec]

@sbruens I added two tests; Please let me know if more tests are needed.