[Rust] Refactor RecordBatchStream trait

[eddyxu]

Refactor to extract RecordBatchStream. There will be two implementation, one is DatasetRecordBatchStream (already implemented), and FragmentRecordBatchStream (to support #557).

[changhiskhan]

lgtm. just curious if the 'static trait bound is needed and if so why does the compiler want it

[eddyxu]

lgtm. just curious if the 'static trait bound is needed and if so why does the compiler want it

seems because stream is used in flat_search

Compiling lance v0.4.1 (/Users/lei/work/lance/rust)
error[E0310]: the parameter type `impl RecordBatchStream` may not live long enough
  --> src/io/exec/knn.rs:56:25
   |
56 |           let bg_thread = tokio::spawn(async move {
   |  _________________________^
57 | |             let batch = match flat_search(stream, &q).await {
58 | |                 Ok(b) => b,
59 | |                 Err(e) => {
...  |
74 | |             drop(tx);
75 | |         });
   | |__________^ ...so that the type `impl RecordBatchStream` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound...
   |
52 |     fn from_stream(stream: impl RecordBatchStream + 'static, query: &Query) -> Self {
   |                                                   +++++++++