Preserve custom metadata when exporting config

[SteveL-MSFT]

PR Summary

Previously, the metadata member of a configuration that was being deserialized only filled in the Microsoft.DSC member and unknown fields were not being preserved. Change is to use the flatten all unknown fields and copy them to the resulting exported configuration.

Also changed some places to use Map instead of HashMap as it preserves order.

PR Context

Fix #668

PR Overview

This PR ensures that any custom metadata fields present in the configuration are preserved during export and updates several type definitions to use ordered maps (Map) instead of unordered HashMaps for consistent ordering. Key changes include:

• Merging custom metadata from the original configuration with newly generated metadata in the export.
• Updating metadata, variables, and references types in configuration and context modules to use Map instead of HashMap.
• Adjusting error messages and metadata field constructions in several functions to reflect these changes.

Reviewed Changes

File	Description
dsc_lib/src/configure/mod.rs	Updated merge logic for metadata and added the new other field.
dsc_lib/src/configure/config_doc.rs	Changed metadata, variables, and parameter types from HashMap to Map.
dsc_lib/src/configure/context.rs	Updated references and variables types to use Map instead of HashMap.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

dsc_lib/src/configure/mod.rs:560

• [nitpick] Consider adding an inline comment to explain the metadata merge logic in invoke_export, clarifying how custom metadata (the 'other' map) from the original configuration is preserved.

match conf.metadata {