[Rust] [Flight] Implement DoExchange on examples

[asfimport]

The gRPC server examples in Rust require all trait members to be exhaustively implemented. The recent DoExchange endpoint to the Flight service is causing failures in Rust.

Reporter: Neville Dipale / @nevi-me
Assignee: Neville Dipale / @nevi-me

PRs and other links:

• GitHub Pull Request #6800

_{Note: This issue was originally created as ARROW-8308. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 6800
#6800