Make it easier to setup a multi-line progress bar

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### Description & Motivation

I'm currently learning ML stuff and made my own model using PyTorch and started using Lightning to wrap it all nicely, because that was suggested to me.

Now when running my training in the terminal, I have my metrics squeezed into the right hand corner of the progress bar, especially when watching more than the "default" loss metric.

I tried changing the RichProgressBar using various LLM assistance, but couldn't get it done in a good way. From my understanding, I would have to rewrite the whole code to allow for a second line, because

### Pitch

I'd like to have an easy way to place the metrics, e.g. last column or new row (with full colspan), and the way I format it.

This could either be a parameter to RichProgressBar or an easy, documented way to overload a function in the class.

### Alternatives

_No response_

### Additional context

_No response_

cc @lantiga @borda

Start by locating the RichProgressBar implementation and its current metric layout. Review how progress-bar columns and metric formatting are configured, then determine an API for placing metrics on another row or column. Done means users can configure a multi-line layout and format metrics without rewriting the progress bar.

Written by the indexing model from the issue text.