feat: add table branch support (#3490)

### Description

Adds first-class support for table branches across the Rust core and the
Python and TypeScript SDKs.

Rust

```rust
use lance::dataset::refs::Ref;

// Create a branch from main and write to it — main is untouched.
let exp = table.create_branch("exp", Ref::Version(None, None)).await?;
exp.add(batches).await?;

// Reopen the branch later: check out from a table, or open it directly.
let exp = table.checkout_branch("exp").await?;
let exp = db.open_table("items").branch("exp").execute().await?;

let branches = table.list_branches().await?;
table.delete_branch("exp").await?;
```

Python

```python
# Create a branch from main and write to it
branch = await table.branches.create("exp", from_ref="main")
await branch.add(data)

# Reopen the branch later: check out from a table, or open it directly.
branch = await table.branches.checkout("exp")
branch = await db.open_table("items", branch="exp")

await table.branches.list()
await table.branches.delete("exp")
```

TypeScript

```typescript
const branches = await table.branches();

// Create a branch from main and write to it
const branch = await branches.create("exp");
await branch.add(data);

// Reopen the branch later: check out from a table, or open it directly.
const checkedOut = await branches.checkout("exp");
const opened = await db.openTable("items", undefined, { branch: "exp" });

await branches.list();
await branches.delete("exp");
```

### Testing
- Added unit tests
- ran smoke tests against python and typescript sdks on local machine


### Next steps
- Add RemoteTable support
- Add Branch Comparison support
- Merge Branching support

python/python/tests/test_namespace.py

@@ -28,6 +28,10 @@ def _ipc_file(table: pa.Table = PUSHDOWN_DATA) -> bytes:
 class _FailingSyncInner:
     name = "hist"
 
+    def current_branch(self):
+        # The pushdown gate only routes server-side when on the default branch.
+        return None
+
     async def schema(self):
         return PUSHDOWN_DATA.schema