Expose Athenz `TokenClient` as public API

Some users may only want to obtain the token to use it with non-Armeria clients.
https://github.com/line/armeria/blob/62a2da22a76f79fd436683c01424e670f3d8d6f8/athenz/src/main/java/com/linecorp/armeria/client/athenz/AccessTokenClient.java#L39
https://github.com/line/armeria/blob/62a2da22a76f79fd436683c01424e670f3d8d6f8/athenz/src/main/java/com/linecorp/armeria/client/athenz/RoleTokenClient.java#L36