Add NON_GCP_PRIVATE_IP_PORT as networkEndpointType for NetworkEndpointGroup

[modular-magician]

This change adds NON_GCP_PRIVATE_IP_PORT as a network_endpoint_type option for the google_compute_network_endpoint_group resource. This is in support for Hybrid Load Balancing.

I added two examples: one with a basic google_compute_network_endpoint_group with the new type, and one with a global external managed load balancer with hybrid connectivity (roughly following the configuration at https://cloud.google.com/load-balancing/docs/https/setting-up-ext-https-hybrid). I also added documentation about the valid combination of this new type and backend service load balancing scheme/balancing modes.

Part of hashicorp/terraform-provider-google#10040

For posterity, this change also sets the instance attribute on the google_compute_network_endpoint resource to be optional, as Hybrid connectivity NEGs use endpoints with just IP and PORT (without an instance). See https://cloud.google.com/load-balancing/docs/negs#hybrid-neg and https://cloud.google.com/load-balancing/docs/https/setting-up-ext-global-https-hybrid#set_up_the_hybrid_connectivity_neg for more details.

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)

compute: add `NON_GCP_PRIVATE_IP_PORT` value for `network_endpoint_type` in the `google_compute_network_endpoint_group` resource

compute: Update `instance` attribute for `google_compute_network_endpoint` to be optional, as Hybrid connectivity NEGs use network endpoints with just IP and Port.

Derived from GoogleCloudPlatform/magic-modules#5684