Support sending schemas for empty streams

[alamb]

Which issue does this PR close?

Closes #3591

Rationale for this change

Some Flight clients expect to see Schema messages even if there is no data (e.g. an empty result set)

What changes are included in this PR?

1. Add with_schema to FlightDataEncoderBuilder to specify schema when creating streams
2. Allow users to retrieve the decoded schema in the FlightDataDecoder
3. Tests

Are there any user-facing changes?

1. FlightDataDecoder now has schema() that returns the actual schema rather than got_schema() which returns a boolean

2. FlightDataEncoderBuilder::with_schema

[tustvold]

Just some minor nits

[viirya]

I'm wondering if it will be more consistent to always require specifying schema instead of two mixed approaches (specifying it by this, getting from first RecordBatch).

[alamb]

I was under the impression that the Flight protocol did not require sending a schema , however the format docs don't seem to specify one way or the other; https://arrow.apache.org/docs/format/Flight.html

Requiring a schema might make the API easier to use correctly with other Flight clients (I am pretty sure the issue we hit upstream in IOx was interoperability with the golang client).

Downstream in DataFusion we almost always have a https://docs.rs/datafusion/16.1.0/datafusion/physical_plan/type.SendableRecordBatchStream.html which has a schema attached.

So in summary, I could go either way. Does anyone else have any thoughts?

[alamb]

FWIW I hit an issue downstream https://github.com/influxdata/influxdb_iox/pull/6711 that makes me think @viirya 's idea to always require a schema is the best (otherwise it is easy to use this interface incorrectly). I'll make another PR shortly

[alamb]

Here is what requiring a schema to encode the stream would look like: #3609

[ursabot]

Benchmark runs are scheduled for baseline = 1afefbb and contender = 902a17d. 902a17d is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
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