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GH-40630: [Go][Parquet] Enable writing of Parquet footer without closing file (#40654)



### Rationale for this change

See #40630

### What changes are included in this PR?

1. Added `FlushWithFooter` method to *file.Writer
2. To support `FlushWithFooter`, refactored `Close` in a way that changes the order of operations in two ways:
   a. closure of open row group writers is now done after using `defer` to ensure closure of the sink, instead of before
   b. wiping out of encryption keys is now done by the same deferred function, ensuring that it happens even upon error

### Are these changes tested?

`file_writer_test.go` has been extended to cover `FlushWithFooter` in a manner equivalent to the existing coverage.

### Are there any user-facing changes?

Only the addition of a new public method as described above.  No breaking changes to any existing public interfaces, unless the two minor order-of-operation changes described above are somehow a problem.

I'm not sure it's a critical fix, but one of the minor changes described above may reduce the likelihood that an attack could inject an error (e.g., an I/O error) to prevent an encryption key from being wiped from memory.

* GitHub Issue: #40630

Authored-by: Peter Newcomb <peter.newcomb@walmart.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>