Engineering: Folder restructuring

Part of #1655

In preparation for further engineering improvements, we will restructure this repository to follow the format demonstrated in https://github.com/jviau/dotnet-worker-layout

```
|- doc // doc: contains all doc files
|- eng // eng: contains all our engineering related items: common targets, ci templates, signing key
|- ci // eng/ci: CI build files and templates.
|- targets // eng/targets: common msbuild .props and .targets and their supporting files.
|- misc // misc: this is a special csproj which collects files, leveraging globbing, to show in Visual Studio, without needing to add one by one to sln.
|- out // out: all project output goes here
|- bin // out/bin: final output
|- obj // out/obj: intermediate output
|- pkg // out/pkg: all packages (nuget or otherwise) go here
|- protos // protos: proto subdirectory *NOTE*: we should consider consuming these as nuget packages.
|- samples // samples: all sample projects.
|- src // src: all source files go here
|- Extensions // src/Extensions: All worker extensions (including ApplicationInsights)
|- Host // src/Host: Functions custom dotnet host projects
|- Shared // src/Shared: "SharedReference" projects, giving us a way to directly share code without using InternalsVisibleTo
|- Worker // src/Worker: All the projects that make up our 'framework'
|- Sdk // src/Worker/Sdk: All the SDK projects
| |- Sdk // src/Worker/Sdk/Sdk: Microsoft.Azure.Functions.Worker.Sdk
| |- Analyzers // src/Worker/Sdk/Analyzers: Microsoft.Azure.Functions.Worker.Sdk.Analyzers
| |- Generators // src/Worker/Sdk/Generators: Microsoft.Azure.Functions.Worker.Sdk.Generators
|- Worker // src/Worker/Worker: Microsoft.Azure.Functions.Worker
|- Core // src/Worker/Core: Microsoft.Azure.Functions.Worker.Core
|- Grpc // src/Worker/Grpc: Microsoft.Azure.Functions.Worker.Grpc
|- test // test: all test code here. Mirrors src structure.
```

To simplify the rest of the engineering work, we will perform this in two phases:

1. Restructure everything as above, _except_ the SDK projects will be in `src/Sdk`.
2. Move SDK projects to `src/Worker/Sdk` after we are ready to combine them into a single release unit.