Ability to configure custom SQL parser dialects on session config

### Is your feature request related to a problem or challenge?

With https://github.com/apache/datafusion-sqlparser-rs/pull/2174, it is now possible to safely derive custom dialects from existing ones, without worrying about breaking hardcoded `dialect_of!` calls (https://github.com/apache/datafusion-sqlparser-rs/issues/1430):

```python
derive_dialect!(MyCustomDialect, PostgreSqlDialect,
overrides = {
supports_from_first_select = true,
},
preserve_type_id = true,
);
```

Datafusion allows configuring the dialect in the [`SessionConfig`](https://docs.rs/datafusion/latest/datafusion/execution/context/struct.SessionConfig.html), however only a [fixed list of values](https://docs.rs/datafusion/latest/datafusion/common/config/enum.Dialect.html) is allowed.

Would it be possible to allow setting our own dialect instance in the config? Perhaps with a `with_dialect()` method on the `SessionConfig`?

There might be limitations I'm missing (e.g. the underlying [`SqlParserOptions`](https://docs.rs/datafusion/latest/datafusion/config/struct.SqlParserOptions.html) might need to be serializable with atomic values, etc).

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

Extending the `RelationPlanner`: https://datafusion.apache.org/blog/2026/01/12/extending-sql/ (doesn't seem applicable).

[Defining a custom SQL parser](https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/sql_ops/custom_sql_parser.rs), but not clear how to configure DF to use it.

### Additional context

_No response_

Read SessionConfig, Dialect, and SqlParserOptions to understand how the fixed dialect values reach SQL parsing. Compare this with the custom_sql_parser.rs example and determine whether a custom dialect can be carried through session configuration. Done should mean a custom dialect can be configured and used by DataFusion's SQL parser without relying on the fixed Dialect list.

Written by the indexing model from the issue text.