feat: add support for schema specification

[cyrixsimon]

Summary:
This PR adds support for optionally specifying a schema in ExportSource::Table and ImportBuilder.
Previously, table names were always unqualified, which caused issues when working with multiple schemas or when the default schema was not sufficient.

Changes:

• Updated ExportSource::Table to accept an optional schema (e.g. Option<&str>).
• Adjusted ImportBuilder to include the schema if specified, resulting in queries like SCHEMA.TABLE instead of just TABLE.
• Ensured backward compatibility — if no schema is provided, behavior remains unchanged.

Closes #40

[bobozaur]

Thanks for the PR!

Looking at this, I think I'd rather take a slightly different approach.

Instead of a single constructor (ExportBuilder::new), let's have two separate ones, new_from_table and new_from_query.

This way, consumers don't have to deal with ExportSource and that can be made internal only and removed from the public API. It would also allow the ExportSource::Table variant to be a struct variant instead of a tuple variant, as that would increase clarity a bit.

The rest would pretty much remain the same.