[SDK-2751] Serialize audience claim when a List

[jimmyjames]

Changes

As discussed in #508, when constructing a JWT using the withPayload method, the aud claim cannot be properly serialized, as we only expected it to be a String array (as it is when using the withAudience method). This change updates the PayloadSerializer to handle both String arrays and Lists to be serialized for the audience claim.

Fixes #508

[yeDor]

Hello @jimmyjames, @evansims, thanks for a fix.
One question on this: why is implementation is aimed to List specifically, and not to the Collection, as it was discussed in #508 ?