Feat/add network connectivity hub spoke acceptance

[AzySir]

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Fixes hashicorp/terraform-provider-google#27458

Part of hashicorp/terraform-provider-google#17109

Adds google_network_connectivity_hub_spoke_acceptance so a hub admin can accept a cross-project NCC spoke (acceptSpoke) and later accept spoke updates (acceptSpokeUpdate) without maintaining auto_accept_projects.

The issue sketched spokes = [...]. This is one spoke per resource; use for_each for several. Same-project spokes are still auto-accepted by the API.

Create calls hubs.acceptSpoke. Setting spoke_etag to the spoke's etag accepts a pending update. Destroy calls hubs.rejectSpoke. google_network_connectivity_spoke now exports etag.

The generated acceptance test creates a second project and is skip_vcr. It needs GOOGLE_ORG and GOOGLE_BILLING_ACCOUNT if run live.

`google_network_connectivity_hub_spoke_acceptance`

networkconnectivity: added `etag` field to `google_network_connectivity_spoke` resource

[github-actions]

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