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GH-36952: [C++][FlightRPC][Python] Add methods to send headers (#36956)



### Rationale for this change

Sending headers/trailers is required for services, but you couldn't do this before.

### What changes are included in this PR?

Add new methods to directly send headers/trailers.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes (new APIs)

* Closes: #36952

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>