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GH-34639: [C++] Support RecordBatch::FromStructArray even if struct array has nulls/offsets (#34691)

### Rationale for this change

A struct array can have a validity map and an offset.  A record batch cannot.  When converting from a struct array to a record batch we were throwing an error if a validity map was present and returning the wrong data if an offset was present.

### What changes are included in this PR?

If a validity map or offset are present then StructArray::Flatten is used to push the offset and validity map into the struct array.  Note: this means that RecordBatch::FromStructArray will not be zero-copy if it has to push down a validity map.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes, RecordBatch::FromStructArray now takes in a memory pool because it might have to make allocations when pushing validity bitmaps down.

* Closes: #34639

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>