Add specialized "target" getters in context interactions

[freya022]

General Troubleshooting

• I have checked for similar issues on the Issue-tracker.
• I have updated to the latest JDA version
• I have checked the branches or the maintainers' PRs for upcoming features.

Feature Request

For discoverability purposes, JDA should have MessageContextInteraction#getTargetMessage() and UserContextInteraction#getTargetUser(), which are the same as getTarget().

It will especially help for UserContextInteraction as it inherits getUser() from Interaction, which is NOT the same as getTarget

Example Use-Case

public void onGetUserAvatar(UserContextInteractionEvent event) {
    final var user = event.getTargetUser(); // same as: event.getTarget()

    // ...
}