Merge scan options into FileIO at TableScan.__init__ time

[rccreager]

Closes #3166

Rationale for this change

The Table.scan(..) method accepts a parameter options: Properties; this parameter is not actually threaded into the FileIO instance used in the DataScan class which scans the data. Instead, the internal callers were using a FileIO instance with only the basic catalog-level parameters set and were not respecting the user-specified options.

This is resolved by merging scan options into the FileIO at TableScan.__init__ time, using the existing load_file_io method. This means that if the user provides new parameters to scan, a new FileIO instance must be created with the merged properties. FileIO properties are immutable after construction, and the cached filesystem objects are built from those properties at first access, so a new instance is required.

If the options are empty or if the key:value pairs are all present in the existing FileIO options, you can use the existing FileIO instance.

I wanted to fix the problem at initialization, rather than at the various callsites, because self.io is also used in scan_plan_helper() for manifest reads, not just in to_arrow() for data reads. Fixing at init time covers both.

Are these changes tested?

I added tests to confirm that the init method has the expected behavior.

Are there any user-facing changes?

Yes. Scan-level options passed to Table.scan(options=...) are now respected by the underlying FileIO during data and manifest reads. Previously these options were silently ignored.