Snippetgen: use f-strings for resource paths

[busunkim96]

This feature was added over a few PRs, but is slated to be reverted in #1134 to move forward enabling snippetgen globally.

The desired end state is a snippet that looks like this (the parent resource is broken up into pieces to help users construct the correct string).

# [START retail_generated_retail_v2_CatalogService_ListCatalogs_sync]
from google.cloud import retail_v2


def sample_list_catalogs():
    """Snippet for list_catalogs"""

    # Create a client
    client = retail_v2.CatalogServiceClient()

    # Initialize request argument(s)
    project = "my-project-id"
    location = "us-central1"
    parent = f"projects/{project}/locations/{location}"

    request = retail_v2.ListCatalogsRequest(
        parent=parent,
    )

    # Make the request
    page_result = client.list_catalogs(request=request)
    for response in page_result:
        print(response)

# [END retail_generated_retail_v2_CatalogService_ListCatalogs_sync]

The previously implemented logic for resource messages cannot handle resource fields that are not directly on the request message. Pub/Sub is one example. The generation fails like this:

leading_comments: " Request for the UpdateSubscription method.\n"
), oneofs=OrderedDict()) has no attribute: 'subscription.topic'
--python_gapic_out: protoc-gen-python_gapic: Plugin failed with status code 1.

https://github.com/googleapis/googleapis/blob/master/google/pubsub/v1/pubsub.proto#L931-L940

// Updates an existing subscription. Note that certain properties of a
// subscription, such as its topic, are not modifiable.
rpc UpdateSubscription(UpdateSubscriptionRequest) returns (Subscription) {
  option (google.api.http) = {
    patch: "/v1/{subscription.name=projects/*/subscriptions/*}"
    body: "*"
  };
}

// Request for the UpdateSubscription method.
message UpdateSubscriptionRequest {
  // Required. The updated subscription object.
  Subscription subscription = 1 [(google.api.field_behavior) = REQUIRED];

  // Required. Indicates which fields in the provided subscription to update.
  // Must be specified and non-empty.
  google.protobuf.FieldMask update_mask = 2
      [(google.api.field_behavior) = REQUIRED];
}

// A subscription resource.
message Subscription {
  option (google.api.resource) = {
    type: "pubsub.googleapis.com/Subscription"
    pattern: "projects/{project}/subscriptions/{subscription}"
  };

  // Required. The name of the subscription. It must have the format
  // `"projects/{project}/subscriptions/{subscription}"`. `{subscription}` must
  // start with a letter, and contain only letters (`[A-Za-z]`), numbers
  // (`[0-9]`), dashes (`-`), underscores (`_`), periods (`.`), tildes (`~`),
  // plus (`+`) or percent signs (`%`). It must be between 3 and 255 characters
  // in length, and it must not start with `"goog"`.
  string name = 1 [(google.api.field_behavior) = REQUIRED];

  // Required. The name of the topic from which this subscription is receiving
  // messages. Format is `projects/{project}/topics/{topic}`. The value of this
  // field will be `_deleted-topic_` if the topic has been deleted.
  string topic = 2 [
    (google.api.field_behavior) = REQUIRED,
    (google.api.resource_reference) = { type: "pubsub.googleapis.com/Topic" }
  ];

  // If push delivery is used with this subscription, this field is
  // used to configure it. An empty `pushConfig` signifies that the subscriber
  // will pull and ack messages using API methods.
  PushConfig push_config = 4;