Support format field in Compute Discovery Doc

Compute is adding a `format` field to the JSON to accompany occurrences of `"type": "any"` in order to specify what the resulting protobuf type should be.

The generator should expect to see them together like:
```
"properties": {
"field_name": {
"type": "any",
"format": "google.protobuf.Value"
}
}
```
This would result in a .proto field definition like:
```
optional google.protobuf.Value field_name = ;
```