feat(python)!: support inserting and upserting subschemas (#1965)

BREAKING CHANGE: For a field "vector", list of integers will now be
converted to binary (uint8) vectors instead of f32 vectors. Use float
values instead for f32 vectors.

* Adds proper support for inserting and upserting subsets of the full
schema. I thought I had previously implemented this in #1827, but it
turns out I had not tested carefully enough.
* Refactors `_santize_data` and other utility functions to be simpler
and not require `numpy` or `combine_chunks()`.
* Added a new suite of unit tests to validate sanitization utilities.

## Examples

```python
import pandas as pd
import lancedb

db = lancedb.connect("memory://demo")
intial_data = pd.DataFrame({
    "a": [1, 2, 3],
    "b": [4, 5, 6],
    "c": [7, 8, 9]
})
table = db.create_table("demo", intial_data)

# Insert a subschema
new_data = pd.DataFrame({"a": [10, 11]})
table.add(new_data)
table.to_pandas()
```
```
    a    b    c
0   1  4.0  7.0
1   2  5.0  8.0
2   3  6.0  9.0
3  10  NaN  NaN
4  11  NaN  NaN
```


```python
# Upsert a subschema
upsert_data = pd.DataFrame({
    "a": [3, 10, 15],
    "b": [6, 7, 8],
})
table.merge_insert(on="a").when_matched_update_all().when_not_matched_insert_all().execute(upsert_data)
table.to_pandas()
```
```
    a    b    c
0   1  4.0  7.0
1   2  5.0  8.0
2   3  6.0  9.0
3  10  7.0  NaN
4  11  NaN  NaN
5  15  8.0  NaN
```

python/python/tests/test_embeddings.py

@@ -198,7 +198,6 @@ def test_embedding_function_with_pandas(tmp_path):
         {
             "text": ["hello world", "goodbye world"],
             "val": [1, 2],
-            "not-used": ["s1", "s3"],
         }
     )
     db = lancedb.connect(tmp_path)
@@ -212,7 +211,6 @@ def test_embedding_function_with_pandas(tmp_path):
         {
             "text": ["extra", "more"],
             "val": [4, 5],
-            "misc-col": ["s1", "s3"],
         }
     )
     tbl.add(df)