Add context function for attributes
Add context function for models
Add context function for schema

- Goal is to have every part of the system be configurable by context

Create Visibility-Type: ENABLED, DISABLED & REQUIRED
Add config option to Attributes which enables or disables attributes by context.

```
model.attr('somevalue', GraphQLString)
  .context(context => this.config({
    create: REQUIRED, // is required during creation
    update: DISABLED, // cannot be set during updates
    where: DISABLED, // will not be used in where & filter input
    order: ENABLED, // can be ordered by
    …
  }))
```

Add config option to Models which enables or disabled model functionalities.

```
model.context(context => {
  this.config({
    findMany: DISABLED,
    findOne: ENABLED,
    findOneArgs: {
      where: ENABLED, // default is REQUIRED (needed for User.findOne-by AuthToken)
      order: DISABLED,
    }
    create: DISABLED,
    update: ENABLED,
    updateArgs: {
      …
    },
    delete: DISABLED,
  })
})
```