Support calling SAFE_CAST via sqlalchemy

**Is your feature request related to a problem? Please describe.**
I could not find a way to use [SAFE_CAST](https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#safe_casting) which returns NULL if the cast fails rather than failing the query.

**Describe the solution you'd like**
A custom `safe_cast` function provided by the library that compiles to `SAFE_CAST`.

**Describe alternatives you've considered**
I can implement a custom `safe_cast` function within my codebase. But, I would have to subclass `sqlalchemy.sql.Cast` (say `SafeCast`) and override the `visit_cast` function in the bigquery dialect to compile `SafeCast` into `SAFE_CAST(...)` instead of `CAST(...).

```python
class SafeCast(Cast):
pass

def safe_cast(col: ColumnElement, type_: TypeEngine)) -> SafeCast:
return SafeCast(col, type_)

# Define custom Bigquery dialect and compiler with
def visit_cast(self, cast, **kwargs):
if isinstance(cast, SafeCast):
return "SAFE_CAST(%s AS %s)" % (
cast.clause._compiler_dispatch(self, **kwargs),
cast.typeclause._compiler_dispatch(self, **kwargs),
)
else:
return super().visit_cast(cast, **kwargs)

```

**Additional context**
Add any other context or screenshots about the feature request here.