feat(schema): Virtual property resolvers

[daffl]

This pull request implements virtual property resolvers as discussed in #2821 and allows selecting all fields - including property resolvers - using the $select query syntax and ensures that databases only get the $select fields they care about. A virtual resolver is essentially a property resolver but does not have a value:

import { resolve, virtual } from '@feathersjs/schema'

const messageResolver = resolve<Message, HookContext>({
  user: virtual(async (message, context) => {
    // Associate the user that sent the message
    return context.app.service('users').get(message.userId)
  })
})

Should close #2595 but we'll leave #2821 open to look at defining dependencies and #2604 to see if something more than $select is needed.

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