update network peering custom routes

[c2thorn]

Fixes: hashicorp/terraform-provider-google#5971
Upstreams/Updates: hashicorp/terraform-provider-google#6020

Updated the original PR to be consistent with general changes made to the providers since the PR was pushed. Added a few other fixes to both the update logic and tests.

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• 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).
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Release Note Template for Downstream PRs (will be copied)

compute: added support for non-destructive updates to `export_custom_routes` and `import_custom_routes` for `google_compute_network_peering`

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[c2thorn]

This is in the third_party directory where the CLA does not apply.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 103 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 103 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=153624"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceSpannerInstance_basic|TestAccComputeNetworkPeering_customRoutesUpdate You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=153643"

[slevenick]

Looks fine, but maybe a couple of those fields should still be ForceNew? Can we actually update any field other than the network field?