fix: Set domain variable as optional on cosmosdb_account module

[balanza]

List of changes

• set domain variable optional on cosmosdb_account module

Motivation and context

The value is documented as optional, but it's actually required.

Type of changes

• Add new module
• Update existing module
• Remove existing module

Does this introduce a breaking change?

• Yes
• No

Other information

Run checks

Useful commands to run checks on local machine

bash .utils/terraform_run_all.sh init local
pre-commit run -a

[balanza]

Besides the fix, I wonder what the domain variable actually means. The name sounds to me like a kind of namespacing or partitioning word, but it's just used to name the cosmos_db_provisioned_throughput_exceeded metric.

Should we remove/rename it?