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GH-40634: [C#] ArrowStreamReader should not be null (#40765)

### What changes are included in this PR?

Small refactoring in the IPC reader implementation classes of how the schema is read in order to support getting the schema asynchronously through ArrowStreamReader and avoiding the case where ArrowStreamReader.Schema returns null because no record batches have yet been read.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

A new method ArrowStreamReader.GetSchema has been added to allow the schema to be gotten asynchronously.

Closes #40634 
* GitHub Issue: #40634

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>