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ARROW-8065: [C++][Dataset] Refactor ScanOptions and Fragment relation

This is the first part of a refactor to make Fragment accessible without a Scan operation instance. This is a breaking change. It introduces the concept of a physical schema and read schema, these concepts are analogous to Avro writer and reader schema.

- Move ScanOptions at Fragment::Scan instead of a property
- Refactor Dataset::GetFragments(ScanOptions) to Dataset::GetFragments(Expression)
- Add Fragment::ReadPhysicalSchema
- Normalize Python's {Dataset,Fragment,Scanner}.{scan,to_table,to_batches}()

Closes #7000 from fsaintjacques/ARROW-8065

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>