[GKE Hub]: Add Fleet default cluster config

[sandmman]

Adds new default cluster config fields to GKEHub resource "Fleet."

b/296461330

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

GKEHub: added `defaultClusterConfig` to `Fleet` resource

[modular-magician]

Hello! I am a robot. It looks like you are a: Community Contributor Googler Core Contributor. Tests will require approval to run.

@melinath, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 3 files changed, 190 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 3 files changed, 190 insertions(+), 2 deletions(-))
TF Conversion: Diff ( 1 file changed, 59 insertions(+))

Missing test report

Your PR includes resource fields which are not covered by any test.

Resource: google_gke_hub_fleet (0 total tests)
Please add an acceptance test which includes these fields. The test should include the following:

resource "google_gke_hub_fleet" "primary" {
  default_cluster_config {
    security_posture_config {
      mode               = # value needed
      vulnerability_mode = # value needed
    }
  }
}

[modular-magician]

Tests analytics

Total tests: 3235
Passed tests 2904
Skipped tests: 330
Affected tests: 1

Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccGKEHub2Fleet_gkehubFleetBasicExample_update

Get to know how VCR tests work

[modular-magician]

$\textcolor{green}{\textsf{Tests passed during RECORDING mode:}}$
TestAccGKEHub2Fleet_gkehubFleetBasicExample_update[Debug log]

Rerun these tests in REPLAYING mode to catch issues

$\textcolor{green}{\textsf{No issues found for passed tests after REPLAYING rerun.}}$

---

$\textcolor{green}{\textsf{All tests passed!}}$
View the build log or the debug log for each test

[sandmman]

@melinath could you please review? I can seem to retrigger a review.

[melinath]

This LGTM - just one question below.

[melinath]

This is a docs-only example; usually we restrict the basic test to only cover required fields - just want to double-check that this is expected to be in most configurations?

[sandmman]

Yes, I think this is the most likely configuration for users using terraform. Defining the fleet resource in terraform is generally worthless without using the default cluster config.

[melinath]

LGTM