Support for external table resources

[serge-bouschet-cko]

Is your feature request related to a problem? Please describe.

No, this is a new feature to support resources such as external tables:
https://docs.snowflake.com/en/user-guide/tables-external.html

Describe the solution you'd like

Enhance the current table resource to support external tables:
https://github.com/chanzuckerberg/terraform-provider-snowflake/blob/main/docs/resources/table.md

Describe alternatives you've considered

We manage them via dbt using operations. A dbt package is available and creates external tables based on source definition: https://github.com/fishtown-analytics/dbt-external-tables
However our wish is now to de-couple these resources from the dbt analytics project and managed them via infrastructure as code.

Additional context

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

[lgwacker]

Was added in this PR #315

[sfc-gh-asawicki]

We are closing this issue as part of a cleanup described in announcement. If you believe that the issue is still valid in v0.89.0, please open a new ticket.