Allow schema to be extendable

[rthideaway]

With a release of webonyx 0.13.1 version there is a feature of extending a graphql schema, see:
webonyx/graphql-php#180. This allows to use "extend" keyword for the types already defined to be extended:

extend type Query {
  newQuery(
    input: InputType
  ): ReturnType
}

This is the feature we'd like to use as our graphql schema is spread across multiple modules in *.gql files and our Schema Plugin is discovering those file and gluing them together. But it's unable to extend the types as there is no Schema extender implemented yet in the graphql module. It's also important to say, that for extending the schema you need to explicitly call SchemaExtender of webonyx library to make it work as shown in example webonyx/graphql-php#180 (comment)

Would be cool to have Extendable schema plugin so this feature can be used

[rthideaway]

Resolved it with #792