Proto: add DataSource/FileSource try_to_proto hook + port FileScanConfig serde #23497

Part of #23494.

Add a try_to_proto hook to the DataSource and FileSource traits in datafusion-datasource (default Ok(None), feature proto), so DataSourceExec rides the ExecutionPlan hook and delegates DataSource → FileSource with no downcast. Port the shared FileScanConfig ⇄ proto conversion out of datafusion-proto into datafusion-datasource (orderings/exprs via the ctx; schema/stats/scalars via datafusion-proto-common). No PhysicalExtensionCodec needed.

Blocks the file-scan and memory-source sub-issues.

See #23494 for the overall pattern and the ctx API. The reference implementation of this hook chain (and the FileScanConfig/FileSinkConfig serde port) is in the full draft #23496. No wire-format change; delete the old central dispatch in the same PR.