nvue-client: Use slightly stronger types for NvueConfig

[DrewBloechl]

Description

Before this, we treated everything inside an NvueConfig as serde_json::Value without caring about its structure. This branch changes a few things about this:

• Add the root-level keys (vrf, router, bridge, etc.) to NvueConfig. Below this the types are still Values but it should reject a lot more obviously-invalid configurations.
• Use different types for NvueConfig and NvueConfigWithHeader. NvueConfig is what the API wants us to send it when applying a new configuration, but it generally responds with NvueConfigWithHeader from its config-related endpoints. The startup YAML file is also a NvueConfigWithHeader.
• Add tests to parse the current agent configs (to be moved to the agent crate later).

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Related Issues (Optional)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

Additional Notes

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