snippetgen: turn resource path strings into f-strings

[busunkim96]

Mock values generated for resource paths currently look like this: (see #941):

async def sample_delete_feed():
    """Snippet for delete_feed"""
    # Create a client
    client = asset_v1.AssetServiceAsyncClient()

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

To make it easier to understand how the string should be filled, it should be turned into a f-string with separate variables in the "initialize request argument(s) section".

async def sample_delete_feed():
    """Snippet for delete_feed"""
    # Create a client
    client = asset_v1.AssetServiceAsyncClient()

    # Initialize request argument(s)
    project = "my-project"
    feed = "my-feed"

    request = asset_v1.DeleteFeedRequest(
        name=f"projects/{project}/feeds/{feed}",