EmbeddingFunction lacks async support, blocks event loop in AsyncTable.add()

## Problem

`EmbeddingFunction` methods (`compute_source_embeddings`, `compute_query_embeddings`) are synchronous-only. When used with `AsyncTable`, this blocks the event loop in production async frameworks like FastAPI.

### Inconsistency between `add()` and `search()`

`AsyncTable.search()` correctly wraps embedding calls in `run_in_executor`:

```python
# table.py – AsyncTable.search()
async def make_embedding(embedding, query):
loop = asyncio.get_running_loop()
return (await loop.run_in_executor(
None, embedding.function.compute_query_embeddings_with_retry, query,
))[0]
```

But `AsyncTable.add()` calls embeddings **synchronously on the event loop** via `_append_vector_columns()`:

```python
# table.py – _append_vector_columns()
col_data = func.compute_source_embeddings_with_retry(batch[conf.source_column])
```

This blocks all concurrent requests when ingesting data with registered embedding functions.

### Broader issue

The `EmbeddingFunction` ABC has no async interface at all. The TypeScript SDK already supports `async computeSourceEmbeddings()` / `async computeQueryEmbeddings()`, but the Python SDK has no equivalent. Most embedding providers (OpenAI, Cohere, Bedrock, etc.) offer async clients that could be leveraged.

## Suggested improvements

1. **Short-term**: Wrap `compute_source_embeddings_with_retry` in `run_in_executor` inside `AsyncTable.add()`, matching the pattern already used in `AsyncTable.search()`
2. **Long-term**: Add `async` variants to `EmbeddingFunction` (`async_compute_source_embeddings`, `async_compute_query_embeddings`) so implementations can use native async HTTP clients instead of thread pool workarounds

## Environment

- lancedb 0.30.2
- Python 3.13
- FastAPI

Start in table.py with AsyncTable.add(), _append_vector_columns(), and the existing AsyncTable.search() run_in_executor path. Then inspect the Python EmbeddingFunction ABC and compare its methods with the TypeScript SDK's async methods. Confirm whether the intended scope is the short-term executor fix, the long-term async interface, or both, and verify that async ingestion no longer blocks the event loop.

Written by the indexing model from the issue text.