chore: Implement AutoCloseable and close() lifecycle for ScalekitClient and ScalekitAuthClient

[coderabbitai]

Summary

Add explicit resource cleanup lifecycle (AutoCloseable / close()) to ScalekitClient and ScalekitAuthClient for correctness in non-singleton / short-lived usage scenarios.

Background

Flagged in PR #64 (#64 (comment)) during the HTTP connection eviction and gRPC keepalive fix.

Currently, ScalekitClient is intentionally used as a long-lived application-lifetime singleton, so OS reclaims resources on JVM exit. However, explicit lifecycle management is a best practice for SDK clients and would support:

• Short-lived or scoped usage (e.g., in tests or serverless contexts)
• Integration with DI frameworks (Spring, Guice) that manage bean lifecycles
• Preventing resource retention in container environments with multiple reloads

Tasks

• Make ScalekitAuthClient implement AutoCloseable; add close() that calls httpClient.close() and shuts down the underlying PoolingHttpClientConnectionManager
• Store the ManagedChannel instance in a private field in ScalekitClient instead of a local variable
• Make ScalekitClient implement AutoCloseable; add close() that shuts down the gRPC channel and closes the auth client

Requested by

@Avinash-Kamath