Change performance group based on FR mode

[adamraine]

#12814 (comment)

We need a way to change and audits group based on the FR mode to progress with
#12814. Specifically, it would be nice to relegate opportunities to the diagnostic group if they can't report any savings.

One option is to use the config to decide the audit group for different modes.

Another option I've been thinking about, optionally return overrideGroup in the audit to dynamically change the group.

[brendankenny]

another option (from #12814 (comment)):

• remove group: 'load-opportunities' and group: 'diagnostics' from the config and have perf audits be in the opportunities section if their audit details are type: 'opportunity', otherwise they're a diagnostic. Would require resurrecting some form of the "hidden" group (e6e4395) to keep the hidden perf audits hidden.

[patrickhulce]

Plan here is to follow @brendankenny 's suggestion of dynamic pulling but closer to v9 as it's fairly close to a breaking change.