feat: add support for RFC6243 with-defaults

[nemith]

Add support for with-defaults which extends , , and with a optional <with-defaults> element.

[Copilot]

Pull request overview

This PR implements support for RFC6243 with-defaults capability in NETCONF operations. The implementation adds an optional <with-defaults> element to <get>, <get-config>, and <copy-config> operations, allowing clients to control how default values are reported by the server.

• Defines WithDefaultsMode type with four RFC6243-compliant mode constants
• Extends Get, GetConfig, and CopyConfig structs with optional WithDefaults field
• Updates XML marshaling to conditionally include the <with-defaults> element with proper namespace

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rpc/with_defaults.go	Defines WithDefaultsMode type, RFC6243 mode constants, and helper struct for XML marshaling
rpc/with_defaults_test.go	Comprehensive tests for XML marshaling of with-defaults in all three supported operations
rpc/rpc.go	Adds WithDefaults field to Get struct and updates MarshalXML to include with-defaults element
rpc/config.go	Adds WithDefaults field to GetConfig and CopyConfig structs with corresponding MarshalXML updates
README.md	Updates RFC6243 status from "planned" to "supported" in the features table
TODO.md	Marks with-defaults as completed in the RFC support checklist

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