Introduce service abstractions and dependency injection in SignTool

[ellahathaway]

Making the changes in https://github.com/dotnet/arcade/pull/16091 was challenging due to the need to adjust many function headers to pass around specific .NET executable paths and related process logic. Additionally, the SignTool project currently has a lot of repeated functionality, especially around running external processes (e.g., invoking msbuild, pkg, tar), locating executables, and managing their arguments.

Proposal

• Refactor the SignTool project to introduce dedicated service abstractions for process execution, pkg, tar, msbuild, and any other external tools.
• Implement these services and register them via Dependency Injection (DI), so dependencies are injected where needed rather than passed through function calls or duplicated logic.
• Consolidate repeated logic (finding executables, running commands, parsing output) into these service classes.

[mmitche]

Agreed overall.

There is some ongoing talk about moving SignTool into the native ESRP tooling. If this happened, I think a complete rewrite would be in order. With, ya know..some real testing.