TPT-3388: Separated GET and POST/PUT options structs where needed

[ezilber-akamai]

📝 Description

Created separate structs where they were previously being reused for both GET and POST/PUT endpoint methods.

Note: I was only able to find one instance of this across the repo but its possible I missed something.

✔️ How to Test

make test-unit
make test-int

[Copilot]

Pull request overview

This PR separates several request option types from response types for interface and firewall rule APIs in linodego.

Changes:

• Adds create/update-specific option structs for Linode interfaces and instance config interface IPv4/default route fields.
• Adds FirewallRuleSetUpdateOptions and updates firewall rules update callers/tests.
• Updates unit and integration tests to use the new option types.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
interfaces.go	Adds create/update-specific nested option structs for Linode interfaces.
instance_config_interfaces.go	Adds create/update-specific VPC IPv4 option structs and conversion usage.
firewall_rules.go	Adds firewall rules update options and changes update signature.
test/unit/interface_test.go	Updates interface update tests to use new default route update options.
test/unit/instance_config_interfaces_test.go	Updates instance config interface tests to use new IPv4 option types.
test/unit/firewall_rules_test.go	Updates firewall rules update test to use the new update options type.
test/integration/nodebalancer_configs_test.go	Updates VPC interface creation setup to use new IPv4 create options.
test/integration/instance_config_test.go	Updates integration setup/update flows to use new IPv4 create/update options.
test/integration/firewall_rules_test.go	Updates firewall rules integration update test to use new update options.

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