JWT aud field is slice of string and is not possible to use as string

[andreadragotta]

Hi,

as i already write in the title the aud field in claims is slice of string but for example in Google Wallet API JWT Sign is necessary to pass a string value.

Is possible to avoid this library constraint.

The JWT rfc https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.3 allow the single string value in special case.

Thanks

[lestrrat]

https://github.com/lestrrat-go/jwx/blob/develop/v2/docs/01-jwt.md#serialize-the-aud-field-as-a-single-string