feat(vpn): add network_config to VPN gateway resource

[itrich]

Exposes networkConfig (predefinedNetworkPrefix, routingTableId) on stackit_vpn_gateway. See API specs for more information.

Description

• Adds network_config (predefined_network_prefix, routing_table_id) to the
  stackit_vpn_gateway resource and data source, letting users attach a custom
  predefined network prefix and/or routing table.
• predefined_network_prefix is RequiresReplace, matching the API's
  constraint that it can't be changed after gateway creation.

Blocker ⚠️

• Works around a live API bug: the generated SDK types
  NetworkConfig.PredefinedNetworkPrefix as []string, but the API actually
  accepts and returns a plain string. Reported upstream at
  predefinedNetworkPrefix must be a string stackit-api-specifications#69. Until that's fixed and a new
  SDK version is released, this PR routes the value through
  AdditionalProperties on the way out and reads the first slice element on
  the way in. Follow-up cleanup commit planned once the spec/SDK fix ships.

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[SerseusWasTaken]

Hello @itrich, thank you for your contribution!

We already have a contribution that addresses this here #1720 and since we already have a internal tracking issue for that PR and it adds a few more features, we will proceed with that one. Therefore i will close your PR as a duplicate, sorry about that. If i missed anything and #1720 implements something different than you did, please feel free to ping me and reopen the PR.

Additionally, thank you for reporting the API issue. I will get into contact with the respective service team to get this fixed ASAP.

Again, feel free to reopen the PR or ping me if you have any concerns.

Best regards,
Jonas from the STACKIT Developer Tools team

[itrich]

No problem @SerseusWasTaken and thank you for making me aware of the pending PR #1720. I'm happy to test this in our local fork (which we need to manage our VPN Gateway).