Fix some compiler warnings in public headers

[erobot]

Motivation

Fix some compiler warnings in public headers when using with -Wextra. These warnings prevent projects using -Wextra -Werror to use pulsar client without modifications.

Modifications

1. Modified public headers to fix compiler warnings
2. Add compile options "-Wextra -Wno-unused-parameter" to compile examples to help find similar issues

Verifying this change

• Make sure that the change passes the CI checks.

This change can be verified as follows:

• Add compile options "-Wextra -Wno-unused-parameter" to compile examples to help find similar issues

Documentation

• doc-required
  (Your PR needs to update docs and you will update later)

• doc-not-needed
  Only the compiler warnings have been fixed, and there is no change in functionality.

• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)

[BewareMyPower]

"Build CPP Client on macOS" failed because it builds with the latest dependency in homebrew. I will remove it from the workflow.