make directives repeatable by default

[nagliyvred]

Fixes #615

Checklist

• Pull requests follows the contribution guide
• New or modified functionality is covered by tests

Description

Currently directive definitions are being ignored when parsing the schema and all directives are being assumed non-repeated. This is causing issues, for example, when using apollo-federation, as it prevents duplicate @key annotations on the federated entity.
While this PR doesn't fix the problem in it's entirety (for example, by parsing directives properly and validating them, as mentioned in #225 ) - it at least allows directive information not to get lost, by assuming all directives could be repeated.

[oryan-block]

Can you check if #645 solves your problem? There was an overwhole of the directives code in GraphQL Java 18

[nagliyvred]

Thanks, will do