Expose Athenz TokenClient as public API

[hyunw9]

Motivation:

AccessTokenClient and RoleTokenClient are currently declared as package-private, preventing users of the Armeria library from directly instantiating or using these classes.

As wrote in #6431, some users may want to obtain only the Athenz token for use with non-Armeria clients.

This PR provides flexibility by exposing these classes as public API, allowing users to fetch tokens directly.

Modifications:

armeria/athenz/src/main/java/com/linecorp/armeria/client/athenz/AccessTokenClient.java

Changed final class AccessTokenClient to public final class AccessTokenClient.

armeria/athenz/src/main/java/com/linecorp/armeria/client/athenz/RoleTokenClient.java

Changed final class RoleTokenClient to public final class RoleTokenClient.

Result:

Closes #6431

After this PR is merged, users will be able to directly import and instantiate com.linecorp.armeria.client.athenz.AccessTokenClient and com.linecorp.armeria.client.athenz.RoleTokenClient.

[github-actions]

🔍 Build Scan® (commit: 56e7e97)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/mw46oacwlzj6m
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/znw5tmpqhltxw
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/aq7eldohstp76
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/dxlqr4ogrpn3g
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/dzzj2ir6nkmpg
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/eldxgcjwdjge2
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/66mqo6gdxgnyy

[ikhoon]

Adding the public keyword doesn’t necessarily mean the user can use it right away.

• TokenClient client is still in private.
• There are no public constructors or builder methods to create AccessTokenClient or `RoleTokenClient.

Suggestion:

• Should we only make TokenClient public class? Let's keep AccessTokenClient and RoleTokenClient package-private.
• Override as method to obtain an instance of TokenClient in AthenzClient?

[ikhoon]

I realized that overriding as is not a feasible solution as AthenzClient.newDecorator() returns a function. Instead, let's add a factory method TokenClient.

public interface TokenClient {

    TokenClient of(....) {
        if (tokenType.isRoleToken()) {
            return new RoleTokenClient(ztsBaseClient, domainName, roleNames, refreshBefore);
        } else {
            return new AccessTokenClient(ztsBaseClient, domainName, roleNames, refreshBefore);
        }
    }