Converter drops Snowflake custom_extensions but Snowflake documentation shows it accepts custom_extensions

Hi there, I noticed that the snowflake converter drops custom_extensions.

However, the Snowflake documentation does show that it accepts custom_extensions. See: https://docs.snowflake.com/en/sql-reference/stored-procedures/system_create_semantic_view_from_ossie_yaml#dataset-fields

Would it be possible to map the custom_extensions now that it is accepted?

```
warnings.warn(
/home/ubuntu/Projects/apache_ossie/converters/snowflake/src/ossie_snowflake/converter.py:512: UserWarning: Dropped from field 'XXXXXXXXXXXXXX' (no Snowflake counterpart): custom_extensions
```

Start in converters/snowflake/src/ossie_snowflake/converter.py around line 512, where custom_extensions is reported as dropped. Compare the converter's field mapping with the Snowflake documentation linked in the issue; done means custom_extensions is mapped and the warning no longer appears for supported input.

Written by the indexing model from the issue text.