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GH-36975: [C++][FlightRPC] Skip unknown fields, don't crash (#36979)



### Rationale for this change

We should skip unknown fields instead of crashing, for forwards compatibility. 

### What changes are included in this PR?

Skip unknown fields in the FlightData deserializer.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #36975

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