Create new schema

Summary:
Our goal is to support hover card and go-do-definition on the _target_ field in a yaml file.
for example, in this file: https://www.internalfb.com/code/fbsource/[eac33136f0d3]/fbcode/content_understanding/projects/fluent2/conf/fluent2_generic.yaml?lines=20
We want to show hover card on
   _target_: content_understanding.framework.training.runner.FitRunner
Also, let users to click through this line to open the python definition of FitRunner.

My plans are: 1. create schema in Glean. 2. Create an indexer and run it regularly. 3. Connect it to Glass via codemarkup schema. 4. implement more downstream application such as diff indexing.

Currently, we only focus on the '_target_' field in a yaml file.
We will add yaml as a new language in Glean.

I followed this wiki: https://www.internalfb.com/intern/staticdocs/glean/docs/schema/fb/workflow/

Reviewed By: iamirzhan

Differential Revision: D65277694

fbshipit-source-id: dfd8daff31f4e882f363987bbde2af4bd55d1480