fix(api): Add custom JSON marshaling for InstancePrototype interface fields

Instance provisioning fails with "Expected only one oneOf fields to be set:
got 0" errors when using InstancePrototype types with interface-typed fields.

Go's default JSON marshaling omits interface-typed fields in certain contexts.
oneOf discriminator fields (image, zone, vpc, primary_network_attachment) are
missing from the JSON payload, causing VPC API validation to fail.

This commit adds custom MarshalJSON methods to all InstancePrototype variant
types that explicitly serialize all fields including interface types.

Adds MarshalJSON() method requirement to InstancePrototypeIntf interface.
This only affects code that implements this interface (SDK-generated types
already comply).

Fixes: #133
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>

# Please enter the commit message for your changes. Lines starting
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# Date:      Thu Oct 23 20:54:50 2025 +0200
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# On branch fix/instance-prototype-json-marshaling
# Changes to be committed:
#	new file:   vpcv1/instance_prototype_marshal_test.go
#	modified:   vpcv1/vpc_v1.go
#	modified:   vpcv1/vpc_v1_integration_utils_test.go
#