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ARROW-8802: [C++][Dataset] Preserve dataset schema's metadata on column projection

Scanner does not preserve the original schema metadata when columns are projected.

Closes #7474 from fsaintjacques/ARROW-8802-dataset-schema-metadata

Lead-authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>