Add data source for retrieving multiple GCE subnetworks

[bestefreund]

Adds a new data source "google_compute_subnetworks", allowing to retrieve multiple GCE subnetworks.

Fixes: hashicorp/terraform-provider-google#18108

Release Note Template for Downstream PRs (will be copied)

`google_compute_subnetworks`

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run.

@slevenick, 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.

google provider: Diff ( 4 files changed, 330 insertions(+))
google-beta provider: Diff ( 4 files changed, 330 insertions(+))

[modular-magician]

Tests analytics

Total tests: 918
Passed tests: 845
Skipped tests: 72
Affected tests: 1

Click here to see the affected service packages

• compute

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

TestAccDataSourceGoogleSubnetworks_basic

Get to know how VCR tests work

[modular-magician]

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

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

---

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

[bestefreund]

Hi @slevenick ,
is here anything further I can contribute?