jwt: Improve allocations (#13532)
jwt: Improve allocations (#13532)

Avoid string -> byte allocations.

```
BenchmarkParseJWTStandardClaims-32       3527152           343.2 ns/op      1489 B/op         21 allocs/op
BenchmarkParseJWTStandardClaims-32       4713199           259.2 ns/op       706 B/op         16 allocs/op

BenchmarkParseJWTMapClaims-32        2666668           448.7 ns/op      1883 B/op         32 allocs/op
BenchmarkParseJWTMapClaims-32        3120709           377.1 ns/op      1227 B/op         28 allocs/op
```
minio/minio,minio/minio,balamurugana/minio,vadmeste/minio,donatello/minio,donatello/minio,krisis/minio,balamurugana/minio,vadmeste/minio,krisis/minio