[Gijs Reijn (Gijsreyn)]

PR Summary

This change:

• Adds command plug-ins: an executable with a *.dsc.plugin.json (or .yaml/.yml) manifest becomes a dsc <command> subcommand.
• Adds dsc plugin list, a "Plug-in commands" section in dsc --help, plug-in names in dsc completer, and dsc schema --type plugin-manifest.
• Adds the plugins.enabled setting to turn plug-ins off, also from the policy file.
• Adds reference docs, schemas, Rust integration tests, 65 Pester tests and a changelog entry.
• Implements phase 1 of the RFC in Add RFC for command plug-ins in DSC #1805. Relates to Add ability to have plugins to dsc.exe #461.

PR Context

The working group asked for a way to extend dsc with commands that ship separately, the way git and the .NET CLI do. The first candidate is dsclint from #1768, which should be reachable as dsc lint without compiling the linter into dsc.

DSC finds plug-in manifests in the same directories as resource and extension manifests, so installing a plug-in is the same as installing a resource. When the first argument isn't a built-in command, DSC looks for a plug-in with that command name, starts its executable with the remaining arguments, and returns its exit code. Standard input, output and error are passed through untouched.

Remarks on the design

• Built-in commands always win. A plug-in can't replace a built-in command or its alias. Such a plug-in is still listed, marked as shadowed.
• Two plug-ins, one command name. The first one found wins and DSC writes a warning that names both. The other is listed as shadowed. This follows Steve's suggestion on the RFC.
• Run by type name. dsc OpenDSC/Lint runs that exact plug-in regardless of who owns the lint command. The match is case-insensitive.
• Context for the plug-in. DSC sets DSC_EXE, DSC_VERSION, DSC_PLUGIN_COMMAND (what the user typed), DSC_PLUGIN_DIR, and forwards the trace level, trace format, progress format and ignore-settings choice as environment variables. Options given before the command name, such as --trace-level, reach the plug-in this way.
• Help. dsc help <command> and dsc <command> --help are forwarded to the plug-in. A mistyped built-in command still gets the "did you mean" tip.
• Disabling. With plugins.enabled set to false, plug-ins aren't discovered or run, and unknown commands fail as they do today.
• Manifest lists. A plugins array is accepted in manifest list files, next to resources and extensions.
• Out of scope. Completion definitions for plug-in arguments (phase 2 of the RFC, waiting on a working group decision), and installing or updating plug-ins.