Refactor extensions code

[SteveL-MSFT]

PR Summary

Apparently Rust allows split implementations of a struct across files for a module. So moved all the extension specific code into separate files. No functional change.

[Copilot]

Pull Request Overview

This pull request refactors the extensions code by splitting the DscExtension implementation across separate files to improve code organization. No functional changes are made to the existing functionality.

Key changes:

• Split extension methods into capability-specific files
• Moved implementation code from dscextension.rs to dedicated files
• Added necessary imports to each new implementation file

Reviewed Changes

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

File	Description
dsc_lib/src/extensions/dscextension.rs	Removed extension method implementations and helper functions, kept core struct definition
dsc_lib/src/extensions/discover.rs	Added discovery capability implementation with imports and helper code
dsc_lib/src/extensions/import.rs	Added import capability implementation with imports and helper code
dsc_lib/src/extensions/secret.rs	Added secret capability implementation with imports and helper code

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