Perform a pre-flight check for basic things before deploying

[saraford]

Before deploying, let's provide a pre-flight check to make sure basic deployment issues are caught ahead of time.

For example:

• check for correct PORT number, e.g. if using a Dockerfile that has port 8051 defined, did the agent specify PORT=8051 to cloud run?
• check for a .gcloudignore file if a .venv or venv folder exists
• etc.

[steren]

In addition to implementing some heuristics on the code, we can also perform an actual "validate only" request on the Cloud Run API, so that the server validates the configuration to be sent is correct (e.g. valid name, region, user has IAM access...).