Prepare querying of graph incides for vertex labels

[farodin91]

My idea first idea to represent indices in grpc.

---

Thank you for contributing to JanusGraph!

In order to streamline the review of the contribution we ask you to ensure the following steps have been taken:

For all changes:

• [ ] Is there an issue associated with this PR? Is it referenced in the commit message?
• [ ] Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• [ ] Has your PR been rebased against the latest commit within the target branch (typically master)?
• [ ] Is your initial contribution a single, squashed commit?

For code changes:

• [ ] Have you written and/or updated unit tests to verify your changes?
• [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [ ] If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• [ ] If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

• [ ] Have you ensured that format looks appropriate for the output in which it is rendered?