Allow overwriting default content source

[farnabaz]

• The default content source (content directory), cannot be overwritten by users. The idea is to allow overwritten the source in content options

defneNuxtConfig({
  content: {
    sources: [
	name: 'content',
        driver: 'some-driver',
	// ...
    ]
  }
})

• For better DX we shall support object syntax for sources option like nitro.storage

defneNuxtConfig({
  content: {
    sources: {
      content: {
        name: 'content',
        driver: 'some-driver',
        // ...
      }
    }
  }
})