[C++][Dataset][Python] Improve ParquetFileFragment serialization

[asfimport]

After ARROW-10131 ParquetFileFragment wraps a FileMetaData, from which all its properties are queried. FileMetaData is emminently serializable, so when pickling a fragment with pre-loaded metadata it would save redundant IO to just serialize the metadata. (An unpickled fragment would then also have pre-loaded metadata.)

#8507 (comment)

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-10435. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
@jorisvandenbossche

[github-actions]

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