Allow python scripting

[lalithsuresh]

With PR https://github.com/vmware/hillview/pull/607, we can use Graal to run different languages on the JVM. Other than the special casing for dates in JSVirtualRowSnapshot, the code is basically guest language agnostic. Any python script that the user supplies can be executed via graal using context.eval("python", "<code>").

That said, graal-python requires some out-of-band rituals to set up, and given how slow it for stock JDKs, the team doesn't seem keen on supporting a maven artifact similar to graal-js: https://github.com/graalvm/graalpython/issues/96

We can keep this around as a tracking issue to see if these problems go away with time.