[WIP] Update opamp-client module to follow style guide

[Copilot]

Update opamp-client module to follow style guide

Analysis completed and identified the following style guide violations to fix:

• Remove final keyword from internal classes (21 files with public final class)
• Add static imports for commonly used utilities like Objects.requireNonNull
• Verify field declarations are marked final where possible
• Check for method parameter/local variable final usage that should be removed
• Verify class organization follows the preferred order
• Apply appropriate visibility modifiers

Files to update identified:

• OpampClientImpl.java, OkHttpSender.java, RetryAfterParser.java, SystemTime.java
• OpampClientState.java, AgentToServerAppenders.java
• Various appender classes (8 files)
• RecipeManager.java, RequestRecipe.java
• HttpRequestService.java, WebSocketRequestService.java
• Delay classes and OpampClientBuilder.java

Fixes #403.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.