GH-50282: [C++][FlightRPC] Refactor GRPC server and transport classes

[Alex-PLACET]

• GitHub Issue: [C++][FlightRPC] Refactor GRPC server and transport classes #50282

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[github-actions]

⚠️ GitHub issue #50282 has been automatically assigned in GitHub to PR creator.

[Alex-PLACET]

@raulcd Can you launch the CI please