Provide deps.edn file

[ieugen]

deps.edn allows for easier consumption and experimentation (on the java side, as a library).

With deps.edn I can depend on a specific pcp git branch and commit.
That way I can experiment with things and dog food features before submitting them upstream.

There is also tools.build that can build the project (and maybe drop the bash script ?! ) .

They can live side by side for a while if you (@alekcz) are not ready to switch to deps.edn .