Warn on env var override

[nullfunc]

On GCP mongodb projects, the gcp cd will create a config/secret (<service_name>_URL) on deployment containing the connection string which the service using mongodb can use to connect.

On first deployment of a GCP mongo db project the connection string config will not yeet exist, so if there is a environment variable using the same name it will not get a warning that it will be override.

Additionally, if there is no environment variable matching the connection string config, one should be added to the service on fixup.

(in-progress work: defang repo - branch: eric/gcp-mongo-connection-string-env-warning)