implement resource google_compute_network_endpoints

[modular-magician]

Implements google_compute_network_endpoints to define multiple endpoints for a zonal NEG in a single resource. The main benefit over google_compute_network_endpoint is that this resource batches calls to attachNetworkEndpoints (and detachNetworkEndpoints), which should drastically speed up attachment/detachment if you have a lot of endpoints. See #14730 for more details.

Initial implementation only allows one of these per NEG, which is probably fine? Separate google_compute_network_endpoint resources should be fine alongside this if needed.

File naming can be changed, it currently mirrors those of google_compute_network_endpoint.

part of #14730

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Release Note Template for Downstream PRs (will be copied)

`google_compute_network_endpoints`

Derived from GoogleCloudPlatform/magic-modules#8068

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