Added the data_source_compute_router_nat

[AarshDhokai]

Fixes : Add a google_compute_router_nat datasource

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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

google_compute_router_nat

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I've detected that you're a community contributor. @slevenick, a repository maintainer, has been assigned to assist you and help review your changes.

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Your assigned reviewer will help review your code by:

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[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, 78 insertions(+))
Terraform Beta: Diff ( 3 files changed, 78 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[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, 78 insertions(+))
Terraform Beta: Diff ( 3 files changed, 78 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2379
Passed tests 2129
Skipped tests: 248
Failed tests: 2

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccContainerCluster_withInvalidGatewayApiConfigChannel|TestAccFirebaserulesRelease_BasicRelease

[modular-magician]

Tests passed during RECORDING mode:
TestAccContainerCluster_withInvalidGatewayApiConfigChannel[Debug log]
TestAccFirebaserulesRelease_BasicRelease[Debug log]

All tests passed
View the build log or the debug log for each test

[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 ( 4 files changed, 151 insertions(+))
Terraform Beta: Diff ( 4 files changed, 151 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2380
Passed tests 2128
Skipped tests: 248
Failed tests: 4

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccContainerCluster_withInvalidGatewayApiConfigChannel|TestAccDataSourceGoogleComputeRouterNat_basic|TestAccDataSourceGoogleServiceAccountJwt

[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[Debug log]
TestAccContainerCluster_withInvalidGatewayApiConfigChannel[Debug log]
TestAccDataSourceGoogleComputeRouterNat_basic[Debug log]
TestAccDataSourceGoogleServiceAccountJwt[Debug log]

All tests passed
View the build log or the debug log for each test

[mitj04]

LGTM