Basic full text search capabilities (#62)

This is v1 of integrating full text search index into LanceDB.

# API
The query API is roughly the same as before, except if the input is text
instead of a vector we assume that its fts search.

## Example
If `table` is a LanceDB LanceTable, then:

Build index: `table.create_fts_index("text")`

Query: `df = table.search("puppy").limit(10).select(["text"]).to_df()`

# Implementation
Here we use the tantivy-py package to build the index. We then use the
row id's as the full-text-search index's doc id then we just do a Take
operation to fetch the rows.

# Limitations

1. don't support incremental row appends yet. New data won't show up in
search
2. local filesystem only 
3. requires building tantivy explicitly

---------

Co-authored-by: Chang She <chang@lancedb.com>

.github/workflows/python.yml

@@ -30,7 +30,7 @@ jobs:
         python-version: 3.${{ matrix.python-minor-version }}
     - name: Install lancedb
       run: |
-        pip install -e .
+        pip install -e ".[fts]"
         pip install pytest
     - name: Run tests
       run: pytest -x -v --durations=30 tests
@@ -49,10 +49,10 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: "3.10"
+        python-version: "3.11"
     - name: Install lancedb
       run: |
-        pip install -e .
+        pip install -e ".[fts]"
         pip install pytest
     - name: Run tests
       run: pytest -x -v --durations=30 tests