Add optional schema support for ExportSource::Table and ImportBuilder

[cyrixsimon]

Problem
Currently, ExportSource::Table and ImportBuilder always use unqualified table names.
In multi-schema environments or when the default schema is insufficient, this prevents specifying the schema explicitly. Using a schema now requires executing an OPEN SCHEMA statement before performing operations on the table.

Proposed Solution

• Allow an optional schema (Option<&str>) for ExportSource::Table
• Update ImportBuilder to include the schema if specified, resulting in queries like SCHEMA.TABLE.
• Preserve backward compatibility — if no schema is provided, behavior remains unchanged.

[cyrixsimon]

Hi, I just wanted to check if you’ve already had a chance to review the feature request.

[bobozaur]

I appreciate your enthusiasm but I mentioned in a separate issue I have my hands quite full right now.

It might take a good while for me to get to the issues you raised. Even then, a release won't happen until #35 is done.

So if you have an absolute need for these features you can use a git dependency instead of a crates.io one in the meantime.