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

We no longer need the random id hack for generating Cloud Run resource
names as the provider now supports a autogenerate_revision_name flag
which allows Google to generate the appropriate resource name for us.

Closes #4

Ignoring changes in name means that one can never run terraform
deployments beyond the initial creation of the webapp service. Terraform
will always modify a service in-place but try to use the same name which
negates the point of *setting* the name in the
google_cloud_run_service.webapp resource.

The downside of this is that one can't then deploy changes without
deleting and re-creating random_id.webapp_revision_name resource but
that is at least documented in the README.

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.