feat(snippetgen): turn resource path strings into f-strings

[busunkim96]

Fixes #965.

Uses the already-existing resource path logic for autogen snippets.

Previous:

    # Initialize request argument(s)
    request = asset_v1.DeleteFeedRequest(
        name="projects/{project}/feeds/{feed}",
    )

Now:

    # Initialize request argument(s)
    project = "my-project-id"
    feed = "feed_value"
    name = f"projects/{project}/feeds/{feed}"

    request = asset_v1.DeleteFeedRequest(
        name=name,
    )

Note: The logic may result in multiple variables with the same name (it doesn't happen for any of the golden APIs, but might for an API with more required fields).

[busunkim96]

I switched to looking through service.resource_messages_dict as the common resources (projects/{project}, folders/{folder}, ...) aren't in api_schema.messages.