feat: add bigframes.bigquery.ml methods (#2300)

This PR adds support for `CREATE MODEL` statement in BigQuery ML via
`bigframes.bigquery.ml.create_model`.
It includes DDL generation logic handling various clauses like
TRANSFORM, OPTIONS, remote models, and different data input formats.
It also refactors `bigframes.core.sql` into a package to support the new
submodule.


---
*PR created automatically by Jules for task
[3846335972146851433](https://jules.google.com/task/3846335972146851433)
started by @tswast*

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Tim Sweña <[email protected]>

Author: google-labs-jules[bot]

bigframes/bigquery/__init__.py

@@ -18,7 +18,7 @@
 
 import sys
 
-from bigframes.bigquery import ai
+from bigframes.bigquery import ai, ml
 from bigframes.bigquery._operations.approx_agg import approx_top_count
 from bigframes.bigquery._operations.array import (
     array_agg,
@@ -157,4 +157,5 @@
     "struct",
     # Modules / SQL namespaces
     "ai",
+    "ml",
 ]