Add unit test for jwt.Sign() variadic parameter options

Originally thought of while reviewing https://github.com/TBD54566975/web5-go/pull/89

We should add a test to `jwt_test.go` something like...

```go
func TestSign_Options(t *testing.T) { ... }
```

Which will test the variadic parameter options. `Type()` will be easy to unit test but `Purpose()` not as straight forward because (I think?) we'll need to generate a DID with multiple keys and instruct the signing to occur on the non-default key (a lot of assumptions in that statement; just briefly looking at it, it's complicated enough to stub in a ticket to investigate)