Add StringIndex support in Python/C++

[markkohdev]

The java bindings currently support a StringIndex class which wraps the Index class but adds a mapping of strings to index integer IDs. This is a handy piece of functionality which would be beneficial and should be fairly straightforward to implement in the Python bindings

[markkohdev]

Additionally, we should support string index containment checking with this python StringIndex as well so that we can write things like

if "some-string-key" in my_index:
  # Do something

[markkohdev]

cc @Ibrokhimsadikov