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GH-38757: [C#] Implement common interfaces for structure arrays and record batches (#38759)

### What changes are included in this PR?

New interface IStructType is implemented by both Schema and StructType.
New interface IArrowStructArray is implemented by both RecordBatch and StructArray.

These changes make it easier to write code which handles both types of structures, which are virtually identical to each other.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

New interfaces have been added to access existing structures.

* Closes: #38757

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