Use cached credentials if they exist

[markeijsermans]

This allows the auth work-around explained here: https://github.com/oSumAtrIX/DownOnSpot/issues/93#issuecomment-2286869696

To use it:

• if existing valid credentials exist, it should still work
• if not, create a new credentials cache file with https://github.com/dspearson/librespot-auth
• replace credentials_cache/credentials.json with the newly generated one