[build] setup public pipeline on dnceng-public

[jonathanpeppers]

This pull request refactors and modernizes the Azure Pipelines YAML configuration for .NET for Android, with a focus on improving maintainability, reusability, and clarity. The main changes involve introducing shared build step templates for macOS and Linux, simplifying the main public pipeline YAML, and cleaning up the Linux build pipeline to use these new templates.

Key changes include:

Pipeline Structure & Modularity:

• Introduced new shared templates: build-macos-steps.yaml and build-linux-steps.yaml, encapsulating the core build logic for macOS and Linux respectively, and removing direct references to private repositories. This allows both public and private pipelines to reuse the same build steps.
• Updated the main public pipeline definition (azure-pipelines-public.yaml) to use these shared templates for the macOS and Linux build stages, and to streamline job definitions.

Linux Build Pipeline Cleanup:

• Refactored build-linux.yaml to delegate all build logic to the new build-linux-steps.yaml template, removing duplicated inline steps and making the pipeline more maintainable.

Build Artifact Management:

• Standardized the copying, packaging, and publishing of build artifacts (such as NuGet packages, symbols, test assemblies, and toolchain pdbs) via the new templates, ensuring consistent artifact handling across platforms.

Pipeline Parameters and Conditions:

• Improved parameterization and variable handling in the main pipeline YAML, including logic to skip test stages for non-fork PRs and to select appropriate agent images and pools for public builds.

These changes make the build pipeline easier to maintain, encourage reuse, and reduce duplication.

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Most important changes:

Pipeline Modularity and Refactoring

• Added build-macos-steps.yaml and build-linux-steps.yaml templates to encapsulate platform-specific build logic, making it reusable and independent of private repos.
• Updated azure-pipelines-public.yaml to use these new templates, simplifying stage and job definitions for public builds.

Linux Build Pipeline Improvements

• Refactored build-linux.yaml to remove inline build steps and instead invoke the new build-linux-steps.yaml template, reducing duplication and centralizing logic.

Artifact Handling Standardization

• Unified the process for copying, deleting, packaging, and publishing build artifacts (including NuGet packages, symbols, test assemblies, and toolchain pdbs) within the new templates for both macOS and Linux.

Pipeline Parameters and Test Skipping Logic

• Enhanced parameterization and variable logic in azure-pipelines-public.yaml to control test execution and agent selection, including auto-skipping tests for non-fork PRs.

[jonathanpeppers]

Example green dnceng pipeline (all tests):

• https://dev.azure.com/dnceng-public/public/_build/results?buildId=1231893&view=results

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