Automate vendoring

[Kielek]

Follow up to #4403

We need to automate vendoring process. It includes

1. Adding new libraries
2. Updating libraries to the newer version
3. Autonotification about new versions

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Some possibility

@Kielek Examples can be found in the dotnet/source-build-reference-packages repo.

1. Vendored libraries as submodules including Newtonsoft.Json
2. Custom project file to build it
3. Example .patch file

Originally posted by @martincostello in #4403 (comment)

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Alternative approach in DD repo: https://github.com/DataDog/dd-trace-dotnet/tree/37030168b2996e549ba23231ae732874b53a37e6/tracer/build/_build/UpdateVendors

[martincostello]

An advantage of submodules is that they can be kept fresh by tools such as dependabot and renovate.