Add support for relative time range `from` and `to` attributes.

[linuspahl]

Description

With #9896 users are able to define an "until" value for the relative time range in the frontend. With these changes we are providing the relative time range for the backend in the following format:

{
  type: "relative",
  from: 300, // 5 minutes ago
  to: 240 // 4 minutes ago
}

The next step is to implement the support the from and to value for the API / backend.
We still need to support the existing structure.

{
  type: "relative",
  range: 300
}