Part of #1010

The goal of this PR is to check-in a package skeleton, laying out files on disk as expected, and establish a baseline based on what the typespec based generator is currently emitting. It isn't expected to be perfect or beta ready, but important to check-in to iterate on top of.

From a review perspective, what I am looking for is the following:

1. Are the various CMakeLists.txt and vcpkg files correct?
2. Is the package name, service name, and folder location as we'd expect?
3. Any feedback on how the files are laid out on disk.
4. Is the CI setup as we'd expect?

The current API shape of the generated client is very much a WIP, and will evolve based on samples and what we expect the shape to be. Hence, it isn't valuable to review the API surface just yet, as part of this PR.

There is related work needed to be done in the emitter, and improvements to the template project folder based on the findings of this exercise. Those are outside the scope of this PR and will be tracked by separate issues.

Here are the hand-written steps taken, using the template project and the generated AppConfig client source files for bootstrapping, including known TODOs and TBD decisions:

1. Start by copying the template project folder

• Rename to appconfiguration, and rename sub-folder to azure-data-appconfiguration.
• Introduce a data sub-folder and rename sub-folders.

2. Modify the header files as needed

• Modify ApiViewSettings.json
  • Update filterNamespace, previewName, serviceName, packageName
• Save a top-level appconfiguration.hpp, with a client, models, and options header file
• Rename rest_client.hpp into configuration_client.hpp and split it into the separate expected headers.
  • Should Models be in a models sub-namespace? TBD design question (Should model types always be in the ::Models sub-namespace or remain in the same parent namespace as the clients, depending on amount of clutter? #6192)
  • Add #includes for local models/options files and remove other headers that aren't needed
  • Simplify #includ headers for options, make ConfigurationClientOptions derive from ClientOptions
  • Options should include the Models header and Client should include both. Models shouldn't need to depend on Options [TBD]
  • Add forward declarations as needed
• Modify dll_import_export.hpp and rtti.hpp
  • Find/Replace _TEMPLATE_ with _DATA_APPCONFIGURATION_ (in both files)
  • Remove unnecessary namespace in dll_import_export.
• Remove template_client.hpp
• Update appconfiguration.hpp root header comment.
• Add doc comments [TODO] - stop gap in place to disable docs generation for this target in CI - [EngSys] Opt outs for faster package iteration #6187

3. Modify the source files as needed

• Modify package_version.hpp namespace
• Rename rest_client.cpp to appconfiguration_client.cpp and update #include headers
• Remove template_client.cpp
• Update HTTP pipeline ctor to include package version
• Save package telemetry name to a shared known constant (_detail::AppConfigurationServicePackageName), to be re-used by multiple clients.
• Split out LROs, PagedResponse, and other implementations into separate source files. [TBD]
• Fix nesting issue with locals being over-written.

4. Update repo root CMakeLists.txt (add_subdirectory(sdk/appconfiguration))
5. Update SDK root CMakeLists.txt (modify project name)
6. Update package root CMakeLists.txt

• Modify the template to remove/add comment and remove duplicative "include" from template
• Add AZ_ALL_LIBRARIES if/else
• Attestation is missing include(CreateMapFile)
• Missing detail about FETCH_SOURCE_DEPS [TODO]
• Modify header and source sets
• Missing BUILD_SAMPLES [TODO]
• Modify test/CMakeLists.txt to be under test/ut.

7. Modify ci.yml

• Added Line and Branch coverage targets of 0 to start with.
• What do you put for CtestRegex?
• Should it be batch: true

8. Modify vcpkg.json file at root of package (the template is outdated)

• What do we use here? version-semver? vs version-string?
• There is inconsistency here, is it even needed?

9. Update NOTICE.txt (rename template to data-appconfiguration)
10. Modify Changelog

• Simplify it to the first version, unreleased, with the required headings

11. cgmanifest [no action]

• Unlike all other SDKs, attestation uses different version of doxygen (1.8.18) from azuresdktooling blob.

12. Modify root README [TODO]
13. Modify vcpkg folder

• Modify Config.cmake.in (renames)
• Modify portfile.cmake (renames)
• Modify vcpkg.json (renames + update core dependency to latest)

14. Modify the test/ut folder and CMakeLists.txt

• Rename azure-template to azure-data-appconfiguration in test CMakeLists.txt
• Link identity and other gtest libraries in test CMakeLists.txt