feat: support snippet generation for services that only support REST transport

[dizcology]

Fixes #1465

This adds support for services that support only the REST transport (Compute). For such services we now generate a sync snippet for every method, for example:

# -*- coding: utf-8 -*-
# Copyright 2022 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# Generated code. DO NOT EDIT!
#
# Snippet for Get
# NOTE: This snippet has been automatically generated for illustrative purposes only.
# It may require modifications to work in your environment.

# To install the latest published package dependency, execute the following:
#   python3 -m pip install google-cloud-compute


# [START compute_v1_generated_AcceleratorTypes_Get_sync]
# This snippet has been automatically generated and should be regarded as a
# code template only.
# It will require modifications to work:
# - It may require correct/in-range values for request initialization.
# - It may require specifying regional endpoints when creating the service
#   client as shown in:
#   https://googleapis.dev/python/google-api-core/latest/client_options.html
from google.cloud import compute_v1


def sample_get():
    # Create a client
    client = compute_v1.AcceleratorTypesClient()

    # Initialize request argument(s)
    request = compute_v1.GetAcceleratorTypeRequest(
        accelerator_type="accelerator_type_value",
        project="project_value",
        zone="zone_value",
    )

    # Make the request
    response = client.get(request=request)

    # Handle the response
    print(response)

# [END compute_v1_generated_AcceleratorTypes_Get_sync]

No changes for services that support gRPC transport, we still generate both sync and async snippets for each method.

Additional changes:

• Ran code formatter on test_samplegen.py. The only updated test is test_generate_sample_spec_basic.

[atulep]

Now method will return "sync" whereas before it was "rest". Will this break anything?

[dizcology]

So far we have not been generating anything for the REST transport so the (incorrect) "rest" segment of the region tag has not been used.

• Generator code that skips over all REST transport snippets: https://github.com/googleapis/gapic-generator-python/blob/main/gapic/generator/generator.py#L157-L160
• Reference region tags in .NET using "sync" and "async" for REST-only Compute API:
  • sync: https://github.com/googleapis/google-cloud-dotnet/blob/main/apis/Google.Cloud.Compute.V1/Google.Cloud.Compute.V1.GeneratedSnippets/AcceleratorTypesClient.GetSnippet.g.cs#L19
  • async: https://github.com/googleapis/google-cloud-dotnet/blob/main/apis/Google.Cloud.Compute.V1/Google.Cloud.Compute.V1.GeneratedSnippets/AcceleratorTypesClient.GetAsyncSnippet.g.cs#L19

[amanda-tarafa]

LGTMing from the side of SnippetGen.

Thanks @dizcology !