[Feature Request] Add configuration for Cloud Run CLI flags

[averikitsch]

Setting flags such as max instance and memory size from the configuration json.

[ahmetb]

Related: was discussed a bit here https://github.com/GoogleCloudPlatform/cloud-run-button/pull/89#issuecomment-541079900

I think overall we need some property bag objects in app.json to control various aspects. Ideally, not a flat list, as that can result in gigantic objects over time as we add more flags to gcloud. So maybe some categorization of such objects can help.

[jamesward]

Mem & cpu options are now available via app.json See: #175

[benwinding]

Would also be great to have the concurrency flag too if possible!

[glasnt]

Many flags were added in #228; there needs to be some thought as to how to make these flags easier to add in the future; maybe directly passing through to gcloud run deploy?