[GCP] Add NAT gateways for outbound public IP addresses

[seankimkdy]

Takes on GCP portion of #434. The NAT gateway requires a cloud router like the VPN gateway we use for multicloud connections. I've chosen to share the router between these two functionalities as GCP says there's no issues as long as BGP is not shared.

You can reuse the same Cloud Router with different network connectivity products. However, each BGP session is unique to the network connectivity product (VLAN attachment, Cloud VPN tunnel, or router appliance instance) that you configure to use with Cloud Router. Different network connectivity products cannot use the same BGP session.

(see https://cloud.google.com/network-connectivity/docs/router/how-to/create-router-vpc-network#establish_bgp_sessions)

Other minor things include a refactor of GCP connectivity test methods.

[github-actions[bot]]

Unit Test Results

310 tests  +1   310 :white_check_mark: +1   3s :stopwatch: -1s  49 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit 305fc58b. ± Comparison against base commit e61c0ba2.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

For the detailed report, please go to Checks tab, click Build and Test, and then download unit_test_coverage artifact at the bottom of build page.

• Your PR branch coverage: 51.5 %
• main branch coverage: 0 %
• diff coverage: 51.5 %

The coverage result does not include the functional test results.

[github-actions[bot]]

For the detailed report, please go to Checks tab, click Build and Test, and then download unit_test_coverage artifact at the bottom of build page.

• Your PR branch coverage: 51.6 %
• main branch coverage: 0 %
• diff coverage: 51.6 %

The coverage result does not include the functional test results.

[github-actions[bot]]

For the detailed report, please go to Checks tab, click Build and Test, and then download unit_test_coverage artifact at the bottom of build page.

• Your PR branch coverage: 51.6 %
• main branch coverage: 0 %
• diff coverage: 51.6 %

The coverage result does not include the functional test results.

[github-actions[bot]]

For the detailed report, please go to Checks tab, click Build and Test, and then download unit_test_coverage artifact at the bottom of build page.

• Your PR branch coverage: 51.6 %
• main branch coverage: 0 %
• diff coverage: 51.6 %

The coverage result does not include the functional test results.