wip: wait cloudrun to be ready

[tmshn]

This is super Work-In-Progress PR for hashicorp/terraform-provider-google#4091 which does NOT work yet.
I'll take holiday for 10 days from tomorrow, so anyone interested feel free to take over my work (otherwise, I'll do this after the vacation)

todos:

• make pr against magic-modules
• fix code to use resource.Retry
  • ref: Wait until container cluster can be operated on. GoogleCloudPlatform/magic-modules#2021
• on initial creation, api response does not seems to have "metadata" field, so we need to distinguish it from an error (as noted in the comment)

discussion point:

As far as I looked around the code, the Decoder function is the only place I can put the logic...but anyone has better idea?

[chrisst]

@tmshn As I suspect we will see more of these kuberenetes style status driven resource I'd love to see this turned into native Magic Modules functionality rather than bolted into the existing decoder. However let me know if you keep working on this and get something working and I'm happy to figure out a way to upstream it into magic modules.

[tmshn]

@chrisst
Well actually...what I want to have initially was mm#2120 (oidcToken field of Pub/Sub), which have seemed to require this CloudRun behavior change.
However, now mm#2440 was merged, mm#2120 is no more needed. Thus, I don't need this PR neither now (currently I'm not using CloudRun).

I close this PR hoping someone takeover this problem.

I might work on this again when it become necessary to me (if there's something I can do).