Refactor metadata into our own classes

[Jefffrey]

Currently the file metadata:

datafusion-orc/src/reader/metadata.rs

Lines 18 to 23 in f19cc7b

	pub struct FileMetadata {
	pub postscript: PostScript,
	pub footer: Footer,
	pub metadata: Metadata,
	pub stripe_footers: Vec<StripeFooter>,
	}

And stripe metadata:

datafusion-orc/src/arrow_reader.rs

Lines 871 to 878 in f19cc7b

	pub struct Stripe {
	pub(crate) footer: Arc<StripeFooter>,
	pub(crate) columns: Vec<Column>,
	pub(crate) stripe_offset: usize,
	pub(crate) info: StripeInformation,
	/// <(ColumnId, Kind), Bytes>
	pub(crate) stream_map: Arc<StreamMap>,
	}

Rely directly on proto structs/types.

Will work on refactoring to add our own versions of these structs/types as a sort of decoupling layer, and to potentially have a nicer interface for what we need from the metadata.