Add support for the Clock load generator source

[bobbyiliev]

Feature request

The materialize_source_load_generator resource currently supports the following types:

• AUCTION, MARKETING, COUNTER, TPCH, KEY VALUE

Materialize now has a clock load generator type which is currently in a private preview:

https://materialize.com/docs/sql/create-source/load-generator/#clock

It will be good to extend the load generator resource so that the clock type is also supported in the Terraform provider.