Improve the direct resource "Acquire" mode

[yuwenma]

Checklist

• I did not find a related open enhancement request.
• I understand that enhancement requests filed in the GitHub repository are by default low priority.
• If this request is time-sensitive, I have submitted a corresponding issue with GCP support.

Describe the feature or resource

Background

Config Connector has an "acquire" mode that users can have config connector to take over and manage an existing GCP resource.

Problems

1. Lack Test Coverage

We don't have (builtin) test coverage that verify each direct resource can be acquired as expected.

2. No code-level clarification

Many contributors and users may not know this feature. Even though the direct tooling and guide supports the "acquire" mode by default, it is very flexible and contributors can make changes for each resource controller (mostly because the corresponding GCP services could be very different). The current tooling does not have a code-level structure that can be read as "acquire a resource should be ...".

Improvements

1. We can add the "acquire" scenario in the current Mock GCP test infra, differentiating it from the create.yaml file. This may not be necessary for each resource, but would be critical for those that have GCP specialties (like create or update is not allowed for Config Connector).
2. We can improve the controller-generator tooling to be more readable, separating the "acquire" edge case handling in the Find, Create, Update, and Delete operations, specifically how "Update" should behave for a first time creation and acquirement.
3. We can add a acquire section in both the direct contributor guide, and add warnings in the google reference doc for any resource that has a special "acquire".

Additional information

No response

Importance

No response

[yuwenma]

cc @acpana Feel free to share your thoughts or assign the issue.