feat: add custom props/targets to customize project build

[filzrev]

This PR add infrastructure to customize build with props/targets on CsProj based toolchain.

Background

Existing implementation have following limitations.

• ImportDirectoryBuildProps/ImportDirectoryBuildTargets are disabled.
  So there is no way to use custom setting when build original benchmark project.
• When using /p: command line parameter. it's handled as global property and it's applied to all dependent projects.
  It need a way to set MsBuild property to specific project.
• Ther is no generic way to add custom MSBuild target to all CsProj based toolchain.

What's changed in this PR

1. Add custom props/targets template files
Following template files are added. (Currently, no settings are contained)

• BenchmarkDotNet.Build.props.txt
• BenchmarkDotNet.Build.targets.txt

These files expected to be used for following purposes.

• Set project specific MSBuild settings.
• Add custom MSBuild target. (e.g. DllGather targets without project build)

2. GeneratorBase.cs
Add GenerateCustomBuildHooksAsync virtual API and invoke on GenerateProjectAsync.
This API is used for generate custom props/targets files that are used on project build.

3. CsProjGenerator.cs.cs
Add GenerateCustomBuildHooksAsync API implementation.

Additionally, some fixed project names are refactored to constant field.

4. Other Templates/*.txt project templates
Add import for props/targets and apply code formatter.