Add `extensions` property to all types and fields configs

[SevInf]

For example:

<?php
return new ObjectType([
    'name' => 'MyType',
    'extensions' => [
        'someTypeProperty' => 'value'
    ],
    'fields' => [
        [
            'name' => 'myField',
            ...
            'extensions' => [
                'someFieldProperty' => 'value'
            ]
        ]
    ]
])

graphql-js did it in graphql/graphql-js#2097.

This is a convenient way to add additional metadata for fields and types and access this data in the resolver. To the extent, it will work right now because 'graphql-php' allows to add custom properties, but in won't work in 100% cases. For example, SchemaExtender class will erase all non-standard properties on the config.

[vladar]

It sounds like a duplicate of #588. Closing this to keep all context in a single issue.