JSON schema importer

[porunov]

Add ability to import schema defined in JSON format via file path or directly via JSON string.
This implementation doesn't migrate or update already defined schema elements. Neither it removes existing defined elements.
SchemaInitStrategy is defined in mind to be able to implement migration and more advance schema management, even so current JSON schema importer implementation is fairly simple. This work is made in hopes to simplify schema definition for beginners, speed-up prototypes development based on JanusGraph, and simplify testing.

Notes to reviewers

Best way to understand how this schema importer work is to read added documentation page docs/schema/schema-init-strategies.md.
By default none of the functionality is affected. New functionality can be activated only by providing non-default values to schema.init.strategy configuration option.

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