TPT-4463: Support RDMA VPC interfaces

[yec-akamai]

📝 Description

Add support to specify VPC type and RDMA VPC. Allow to create the RDMA intercase with the instance creation. Allow to update and delete the RDMA interface with the standalone interface endpoints.

✔️ How to Test

make test-unit

[Copilot]

Pull request overview

This PR adds first-class support for GPUDirect RDMA VPCs by introducing a VPC type enum, surfacing vpc_type on VPC/subnet responses and VPC create requests, and enabling RDMA VPC interface configuration via instance-create and interface update endpoints.

Changes:

• Add VPCType (regular / rdma) and plumb vpc_type through VPC and subnet models + VPC create options.
• Add RDMA VPC interface models/options (rdma_vpc) and support specifying RDMA interfaces during instance creation via a new LinodeInstanceInterfaces field.
• Add unit tests + fixtures covering RDMA VPCs and RDMA interfaces.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
vpc.go	Introduces VPCType and adds vpc_type to VPC + VPC create options.
vpc_subnet.go	Adds vpc_type to VPCSubnet model for responses.
regions.go	Adds a new region capability constant for GPUDirect RDMA.
interfaces.go	Adds rdma_vpc to interface models and adds RDMA create/update option structs.
instances.go	Adds LinodeInstanceInterfaces and extends marshaling conflict checks to support RDMA interface creation on instance create.
test/unit/rdma_vpc_test.go	Adds unit tests for RDMA VPCs and RDMA interface behavior, including instance-create marshaling.
test/unit/fixtures/*.json	Adds fixtures for RDMA VPC/interface unit tests.

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