Add joins in /meta

I am building a data table editor for the end user. I want the end user to be able to pick the desired columns he wants to see in the table.

To do so, I want the user to select a cube first, then to pick from the related dimensions, as well as join dimensions, he wants to see as columns.

I can get the cubes and related dimensions with `/v1/meta` but it is missing the `joins`.

**The solution**
` /v1/meta` endpoint to return joins as well for each cube.

With joins, we could aggregate dimensions for a desired Cube with the ones from its joined cubes.