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GH-37884: [Swift] allow reading of unaligned FlatBuffers buffers (#38635)

The PR enables the swift readers to read from unaligned buffers (fix for issue: 37884)

Enabling unaligned buffers incurs a performance penalty so the developer will need to consider this when enabling this feature.  

It is not currently possible to recover from a buffer unaligned error as this error is a fatalError so trying aligned and then falling back to unaligned is not an option.  Also, FlatBuffers has a verifier that should be able to catch this error but currently it seems to fail on both aligned and unaligned buffers (I tried verifying the example python server get return value but verification fails even though the buffers are able to be read successfully)
* Closes: #37884

Authored-by: Alva Bandy <abandy@live.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>