Sometimes we don't need a SQL instance for the webapp. Allow
sql_instance_connection_name to be empty and, if so, don't add the Cloud
SQL connection roles to the service account or add the SQL instance
annotation to the webapp.

Closes #5

As noted in #6, we were hard-coding the service account id used for the
webapp to "webapp-run". This meant it was impossible to deploy more than
one webapp in a project.

Form a better default from the "name" variable. For existing deployments
the service account id will be unchanged if the "name" variable is at
its default value.

Allow the service account id to be specified explicitly via the
optional "service_account_id" variable.

Closes #6

Allow an optional domain mapping to be configured for the webapp. If a
domain mapping is configured, appropriate DNS records are provided as
outputs.

Closes #2

This module manages a Cloud Run-hosted container. It takes care of making
sure the container is connected to a Cloud SQL instance and sets
environment variables on the application.