Add rule_custom_name variable to PSC endpoints bare targets

[crrlcx]

This pull request updates the naming convention for Private Service Connect (PSC) resources in the psc_forwarding_rule Terraform module. The main change is to allow more flexible and descriptive custom names for PSC compute addresses and forwarding rules by introducing a new optional variable, rule_custom_name, which defaults to "custom".

Naming improvements for PSC resources:

• Added a new optional variable rule_custom_name to the psc_endpoints variable definition, allowing users to specify a custom name base for PSC resources.
• Updated the google_compute_address.psc_address and google_compute_forwarding_rule.psc_forwarding_rule resources to use rule_custom_name (with the index as a suffix) instead of a hardcoded "custom" base when neither a CloudSQL nor AlloyDB instance name is present. [1] [2]

[google-cla]

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