Remove duplicate datafusion code

[timsaucer]

OneShotPartitionStream is defined in two places

• rust/lance-datafusion/src/exec.rs
• rust/lance/src/datafusion/dataframe.rs

In addition to this struct, there is a SessionContextExt defined in each of these. From a dependency graph it looks like lance-datafusion is the place we should have a single definition of these and then remove references to the other. The one issue I see is that the SessionContextExt in lance is more complete than the one in lance-datafusion.

[timsaucer]

More generally, how would people feel about upstreaming the OneShotPartitionStream? I think it's very handy.

[Xuanwo]

More generally, how would people feel about upstreaming the OneShotPartitionStream? I think it's very handy.

Seems a good idea, I will start a PR on this.