[AIP-193] Allow Dynamic Variable Placeholders in Error Message

[yordis]

As a programmer, I would like to define a placeholder in the error message to have static error messages with Dynamic Variables capabilities.

Example:

{
  "error": {
    "code": 429,
    "message": "The zone '{zone}' does not have enough resources available to fulfill the request. Try a different zone, or try again later.",
    "status": "RESOURCE_EXHAUSTED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "RESOURCE_AVAILABILITY",
        "domain": "compute.googleapis.com",
        "metadata": {
          "zone": "us-east1-a",
          "vmType": "e2-medium",
          "attachment": "local-ssd=3,nvidia-t4=2",
          "zonesWithCapacity": "us-central1-f,us-central1-c"
        }
      }
    ]
  }
}

Rationale

Take #1110 as an example of an error-rune situation; it could be just a simple mistake or an honest mistake we make more often than it needs to happen.
In the worst case, the developer used a different value format between the metadata value and the message dynamic section.

Promoting dynamic variable placeholder allows "static" error messages, which are much easier to deal with, without losing the capability of getting a final Displayable Error Message that tools could decide to print with the replaced values.