feat(snippetgen): generate mock input for required fields

[busunkim96]

Generate mock field inputs for all required fields.

• For oneofs, the first option is selected.
• For enums, the last option is selected (as 0 is often unspecified).
• Mock input for string fields that reference resources (resource_pb2.resource_reference) is the pattern provided in the protos.

TODOs:

• When a resource's pattern is * this results in unhelpful strings

  message Asset {
    option (google.api.resource) = {
      type: "cloudasset.googleapis.com/Asset"
      pattern: "*"
    };

  request = asset_v1.BatchGetAssetsHistoryRequest(
     parent="*",
  )

• Map fields are not yet handled.

Other changes:

• Fields are set like foo.bar rather than through dict access foo["bar"]. This is because the former allows you to set fields nested more than one field deep foo.bar.baz.

  Before:

  output_config = {}
  output_config["gcs_destination"]["uri"] = "uri_value" # fails as there is no nested dict

  After:

  output_config = asset_v1.IamPolicyAnalysisOutputConfig()
  output_config.gcs_destination.uri = "uri_value"

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• tests/snippetgen/goldens/mollusca_generated_mollusca_v1_snippets_one_of_method_async.py:26, tag mollusca_generated_mollusca_v1_Snippets_OneOfMethod_async
• tests/snippetgen/goldens/mollusca_generated_mollusca_v1_snippets_one_of_method_sync.py:26, tag mollusca_generated_mollusca_v1_Snippets_OneOfMethod_sync

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[leahecole]

Okay @busunkim96 - I looked at some - didn't make it through all, but a lot of my comments will apply to multiple samples, I think. Hope it was at least a little helpful!

[leahecole]

I'm not familiar with this syntax - is this asterisk something that's typical for this service?

[busunkim96]

The * wouldn't actually work if passed to the service. The snippetgen logic pulls the first resource pattern out of the protos, and in some uncommon cases it is just a * rather than some more specific pattern like projects/{project}. The * pattern creates problems for generating resource helper methods and tests too - see #701

[busunkim96]

@software-dov Thank you for the review! PTAL

[software-dov]

Mostly nitpicking and one minor concern, looks great! Thanks for doing this, and I hope the existing samplegen code wasn't too unpleasant to work with.