[C++] Added support for installing dependencies with Conan

[mkmkme]

What is the purpose of the change

With this small change, developers of C++ bindings of Avro can build it on a machine with no boost installed.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? no
• Although the new possible way of building is documented in README

[martin-g]

You need to add the license header for the new file.

[mkmkme]

Actually now I'm not entirely sure if I want this file into the repo or not 😅 I'm used to being a big conan's advocate and I still use it in situations like this.

However, with this project I started using nix-shell instead and it works just fine as well.

So I think I'll close this PR for now. If needed, I'll easily reintroduce either conanfile, or nix-shell file, or any other alternative to it.