Handle extended input

[lburja]

Resolves #353

Checklist

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

Description

The GraphQL spec allows extend input, but the graphql-java-tools library did not handle this case.

This commit, enables the following to work:

type Mutation {
    save(input: UserInput!): Boolean
}

input UserInput {
    name: String                        
}

extend input UserInput {
    password: String
}

# and later on:
# mutation {
#    saveUser(input: {name: "John", password: "secret"})
# }

[vojtapol]

Thank you for your contribution!